* Thank's to PirateMike who had posted the informations until lvl 200 on the FS Forum. *

A common question I see is “what skills should I get”?
I hesitate to give definitive answers because the skills you should get are dependant on your playing style.
Someone who loves to PvP will have a different set of skills than those of a person who wants to simply level up.
So to help with making a decision that’s right for YOU,
I have decided to explain what the skills are in detail, and to explain where I find them are useful.

Skills fall in three major categories:

When you start, the skill point cap is 100.
Every 50 levels after that, the cap raises 5 points.
So at level 50 your cap is 105, at level 100 the cap is 110,
and so on up to skill level 150 which you gain at character level 500.
Skill descriptions taken from the game are in black, quoted directly from the game.
Notes in blue are from the original post via Forum.
Cost is how many stamina points it costs to cast this buff,
Duration is given in minutes.

Offensive skills:
Level 1:
Rage: [Cost 10, Duration 90]
+0.2% to base attack per point. This skill applies to everything you attack.
Notice it says BASE attack. It doesn’t apply to any guild or armor/weapon bonuses.
In my opinion, the bonus given by your equipment far outweighs the 20% to your base attack this skill would give.
 

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  Stun: [Cost 15, Duration 90]
  +0.1% chance per point to halve your opponents chance to hit.
  Again, this is a multi-target skill. This skill (at 100), give you a 10% chance to reduce your opponent’s attack stat by half.
  Not the base stat, but the whole shebang. At first look, this looks appealing, but as you get in to the higher levels (lvl 50 +),
  you are looking to one-hit kill your mobs, so it becomes a moot point.
   
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  Level 25:
  Fury: [Cost 10, Duration 90] {10 point pre-requisite in Rage}
  +0.1% base Attack and +0.1% base Damage per point.
  An all around buff, this skill again seems like a good deal. And it isn’t all that bad.
  Of all the ‘base stat’ skills, this isone of the more useful.
  Most players level up their attack and damage points while leaving their armor, defense, and HP leveling alone.
  But because it is a base bonus, it has a very limited usefulness.
  
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  Bloodthirst: [Cost 10, Duration 45] {10 point pre-requisite in Rage}
  +0.2% chance per point to drain 5% of your opponents current HP per combat turn from your opponent.
  This skill is only useful in multiple rounds of combat, and you don’t receive that HP that’s drained from your opponent.
  Since we are trying to avoid multiple rounds of combat… Not a worthy skill.
   
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  Enchant Weapon: [Cost 10, Duration 90]
  +0.1% per point stat bonus increase to your equipped weapon (Excludes 'Gain' bonuses).
  This is handy depending on your weapon. If your weapon only has one stat bonus, okay.
  But if your weapon has multiple stat bonuses, better. ‘Gain bonuses’ refer to a + of gold, stamina, or exp hourly gain.
  This is the first worthy skill in this category in my opinion.
  
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  Level 75:
  Berserk: [Cost 15, Duration 90] {10 point pre-requisite in Rage & Fury}
  +0.2% base Damage per point.
  Again, this is another base stat bonus, probably more useful than Fury.
  With a lot of players putting large amounts of level up points in Damage, this can be very handy.
   
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  Holy Flame: [Cost 15, Duration 90] {10 point pre-requisite in Enchant Weapon}
  +0.2% extra damage vs. undead per point.
  This is the first creature-specific skill.
  Out of 393 creatures listed in FallenGuide, only 40 are classified as undead as of 9/19/07. ‘Nuff said about that.
  
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  Level 150:
  Dark Curse: [Cost 20, Duration 60] {10 point pre-requisite in Rage, Fury & Berserk}
  +0.2% reduction of opponents defense per point.
  More useful at higher levels than lower,
  as this affects the opponent’s entire defense stat and 20% of a mob with a defense of 12 is only 3 points (rounded up).
  
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  Level 200:
  Shockwave: [Cost 20, Duration 90]
  +0.1% chance per point that your opponent will forfeit their next combat turn.
  I have heard higher-level players say that one-hitting mobs is almost impossible at these levels, so I imagine this skill could be useful.
  
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  Ignite: [Cost 10, Duration 60]
  +0.1% chance per point that your opponent will be set on fire.Shockwave
  Each successful hit thereafter will inflict between 5% and 10% extra damage.
  This is an automatic damage booster.
  
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  Level 250:
  Super Elite Slayer: [Cost 25, Duration 15]
  +0.2% per point reduction of damage, attack, defence and armor to super elite creatures.
   
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  Wither: [Cost 15, Duration 60]
  +0.2% per point chance of a 50% reduction of your opponents HP at the start of combat.
  
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  Level 300:
  Shatter Armor: [Cost 20, Duration 60]
  +0.05% per point chance to reduce opponents armor by 75%.
   
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  Death Wish: [Cost 20, Duration 45]
  +0.03% per point chance to instantly kill vs. creatures. (Excludes Super Elites)
   
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  Spell Breaker: [Cost 35, Duration 45]
  +0.1% per point chance to remove a random buff from PvP target upon a successful attack.
   
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  Level 400:
  Spectral Knight: [Cost 15, Duration 45] {10 point pre-requisite in Shatter Armor}
  +0.1% per point chance to reduce targets armor by 100%. (vs Creature only)
   
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  Keen Edge: [Cost 10, Duration 60] {10 point pre-requisite in Death Wish}
  +0.1% per point to your attack for each complete set equipped.
   
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  Level 500:
  Arterial Strike: [Cost 20, Duration 60] {10 point pre-requisite in Shatter Armor & Spectral Knight}
  Gain additional 0.1% xp per point for every additional round of combat. (Note that this does not activate if conserve activated)
   
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  Death Dealer: [Cost 20, Duration 60] {10 point pre-requisite in Death Wish & Keen Edge}
  For every 5 kills in a row, without dying, you gain +0.01% extra damage per point (Max 20% and vs. creatures only).
   
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  Level 600:
  Savagery: [Cost 15, Duration 45] {10 point pre-requisite in Death Wish, Keen Edge & Death Dealer}
  0.05% chance per point that your defense stat is added to your attack and your armor stat is added to your damage.
   
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  Level 700:
  Chili Strike: [Cost 20, Duration 90] {10 point pre-requisite in Death Wish, Keen Edge, Death Dealer & Savagery}
  0.1% per point of your Health total is added to your damage
   
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  Shield Strike: [Cost 20, Duration 45] {10 point pre-requisite in Death Wish, Keen Edge, Death Dealer & Savagery}
  0.1% per point chance that your defense stat is reduced to zero and your damage is doubled.
   
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  Defensive Skills:
  Level 1:
  Great Vigor: [Cost 10, Duration 90]
  +0.2% base HP per point.
  I know too many players that never put any level up points into HP, so I would skip this one.
   
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  Level 25:
  Fortify: [Cost 10, Duration 120]
  +0.1% base Armor per point.
  In my opinion, another useless skill as there isn’t an opportunity for your defense to come into play if you are one-hitting the mob.
   
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  Evade: [Cost 10, Duration 90]
  +0.1% base Defense per point.
  See Fortify.
   
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  Absorb: [Cost 20, Duration 120]
  +0.1% chance per point that you will absorb 25% of the damage inflicted on you.
  Ok, at level 100, that’s 10% chance you are going to absorb 25% of the damage IF the mob hits you.
   
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  Level 75:
  Rock Skin: [Cost 15, Duration 90] {10 point pre-requisite in Fortify}
  +0.1% base Defense and +0.1 base Armor per point.
  Limited by the ‘base’ side of this skill, this buff has very limited usefulness.
   
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  Enchanted Armor: [Cost 10, Duration 90] {10 point pre-requisite in Absorb}
  +0.1% per point stat bonus increase to your equipped armor. (Excludes 'Gain' bonuses).
  This is a useful boost, as not everyone can afford the “prime” equipment and the bonus applies to the armor, not the base stat.
   
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  Level 150:
  Aura of Protection: [Cost 20, Duration 90]
  +0.1% base Defense, +0.1% base Armor and +0.1% base HP per point.
  Of the base stat defense buffs, this is the one to get if you want one. But that is based entirely on that it boosts 3 stats.
   
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  Deflect: [Cost 25, Duration 300]
  +0.25% chance per point that a player attacking you will automatically fail before combat starts.
  This is a PvP-only skill, with very little usefulness.
  Attackers automatically gain the first attack which would make it useful, but for the buff length.
  5 hours is longer than most players log off, and at the level you gain this buff most players don’t PvP.
  If they ever extend the duration of this buff to 12 or 24 hours, it would be worthy.
   
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  Level 200:
  Force Shield: [Cost 10, Duration 60]
  +0.1% chance to reduce damage done to you to 1.
  Another nice skill, as it applies to not only PvM but also to PvP combat. Useful if you are in a pinch for armor defense.
   
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  Unbreakable: [Cost 20, Duration 90]
  +0.5% chance per point of equipment not taking durability loss during combat.
  If you want to save a few pennies on repairing your armor, this is the buff for you. It’s a good buff, but there are more pressing buffs to get.
   
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  Level 250:
  Assist: [Cost 30, Duration 120]
  +0.05% per point chance of one of your allies assisting in combat vs. creatures.
  (Ally is randomly selected and adds 50% of their attack, defense, damage, armor and hp - note this also excludes allies whom are more than 25 levels above you.)
   
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  Level 300:
  Constitution: [Cost 25, Duration 30]
  +0.1% per point increase to your defense.
   
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  Level 400:
  Conter Attack: [Cost 20, Duration 60]
  Uses 0.25% extra stamina (per point) to add 0.25% to both attack and damage. (Both values are rounded up, vs. creature only)
   
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  Summon Shield Imp: [Cost 50, Duration 60] {10 point pre-requisite in Constitution}
  Creates an Imp which can absorb 100% of damage. Each full absorb uses one of the Shield Imp's hit points.
  The Shield Imp starts with 3 hit points and gains one for each 50 points placed in this skill. T
  he Shield Imp auto-debuffs when it reaches zero hit points.
  (Note Super-Elites can crush the imp in a single turn regardless of hit points remaining and it only works in PvE.
   
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  Level 500:
  Vision: [Cost 20, Duration 90]
  Lights up dark realms. More skill points allow more vision on the 'Map' screen.
  Vision radius increases every 50 levels
   
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  Fortitude: [Cost 15, Duration 90] {10 point pre-requisite in Constitution & Summon Shield Imp}
  Defense stat is added to HP. (0.1% per point)
   
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  Level 600:
  Flinch: [Cost 20, Duration 60]
  0.1% per point decrease in enemies Attack stat
   
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  Level 700:
  Terrorize: [Cost 20, Duration 60] {10 point pre-requisite in Flinch}
  0.1% per point decrease in enemies Damage stat
   
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  Nightmare Visage: [Cost 15, Duration 90] {10 point pre-requisite in Constitution, Summon Shield Imp & Fortitude}
  0.25% per point of your Attack will be transferred into Defense. (Great for offline protection!)
   
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  Special Skills
  Level 1:
  Find Item: [Cost 10, Duration 60]
  +0.1% per point increase of creatures current drop rate.
  This is primarily useful when hunting for a specific item. Also very handy during legendary events.
   
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  Treasure Hunter: [Cost 15, Duration 120]
  +0.2% per point additional gold from creatures.
  This is a good skill, as it is an automatic increase in the amount of gold gained.
   
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  Deep Pockets: [Cost 10, Duration 90]
  +0.25% per point reduction in gold lost on failed combat vs. creatures.
  This is almost a defensive skill, useful if you are having a rough time one-hitting the mobs.
   
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  Quest Finder: [Cost 5, Duration 90]
  Increases the chance a quest item will drop.
  (If you fail to obtain an item, an extra roll is given for Quest Finder at a fixed percentage based on the points allocated to the skill.
  If this second roll is successful, you will obtain one of the available quest items drops (if any)).
   
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  Level 25:
  Adept Learner: [Cost 10, Duration 90]
  +0.2% per point increase in xp from creature kills.
  This is the first “must have” skill for leveling, giving you an automatic boost in exp gained.
  It would be wise to save for this skill to have the second you make lvl 25.
   
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  Defiance: [Cost 15, Duration 120]
  +0.25% per point reduction in xp lost when defeated in combat vs. creatures.
  See Deep Pockets (above).
   
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  Level 75:
  Librarian: [Cost 10, Duration 60]
  +0.1% per point chance to gain double xp from creatures.
  Another skill that’s good for leveling. It isn’t an automatic boost to exp gained, but it does hit.
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  Merchant: [Cost 10, Duration 60]
  +0.05% per point chance to gain double gold from creatures.
  This is the “gold gain” version of Librarian.
   
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  Level 150:
  Last Ditch: [Cost 15, Duration 120]
  +0.2% per point chance to survive death in combat (once per combat).
  I doubt this skill would be very useful even at level 115. Your equipment should be keeping you ‘alive’ even if you aren’t one-hitting.
   
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  Level 200:
  Animal Magnetism: [Cost 10, Duration 60]
  +0.2% per point chance to make certain creatures re-spawn at your location.
  This skill is useful for areas where the mobs are sparse and requires a lot of walking to find them. Only re-spawns mob types you kill.
   
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  Empower: [Cost 20, Duration 60]
  +0.1% per point increase to all currently active enhancements.
  Enhancements are those things on the right side of the item description box &
  total enhancements are listed on the left side of your main character page.
  This is the only buff that impacts your enhancements.
   
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  Doubler: [Cost 5, Duration 120]
  At skill level 50+, 2x Stamina usage in combat in return for 2x gold/xp.
  At level 100+ 3x, and at level 150+ 4x. Note that stamina, gold, and xp losses are normal (not multiplied) if you lose a battle.
  This is a good skill for areas with low monster content, especially coupled with Animal Magnetism.
  Level 250:
  Conserve: [Cost 10, Duration 45]
  +0.05% per point chance that combat (vs. players and vs. creatures) will use no stamina.
  (Excludes group/relic combat)
   
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  Brewing Master: [Cost 10, Duration 30]
  +0.5% per point to the duration of potions when consumed while active.
   
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  Four Leaf: [Cost 20, Duration 60]
  +0.1% per point chance that craftable items are discovered already 'Perfect'.
   
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  Level 300:
  Extend: [Cost 10, Duration 45]
  +0.25% per point increase to skills durations that are cast while this skill is active.
   
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  Level 400:
  Inventor: [Cost 15, Duration 60]
  Increases chance of success when attempting to Invent items/potions.
  (A fixed +0.05% chance per point extra chance of success)
   
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  Extractor: [Cost 15, Duration 60]
  Increases chance of success when attempting to extract Components from Resources.
  (A fixed +0.05% chance per point extra chance of success).
   
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  Level 500:
  Inventor II: [Cost 20, Duration 60] {10 point pre-requisite in Inventor}
  Chance not to consume (or consume less) components when inventing items.
   
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  Buff Master: [Cost 10, Duration 60] {10 point pre-requisite in Brewing Master}
  0.2% per point chance to half the stamina cost (rounding up) when casting skills on other players. (Does not work on self!)
   
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  Level 600:
  Reflection: [Cost 10, Duration 90]
  0.1% per point of enemies damage inflicted is added to your next combat strike.
   
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  Level 700:
  Light Foot: [Cost 15, Duration 120] {10 point pre-requisite in Conserve}
  0.05% chance to use no stamina while moving on the world map.
   
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  Mesmerize: [Cost 20, Duration 60] {10 point pre-requisite in Reflection}
  0.1% per point chance to reduce a creatures armor and defense by 50% (vs. creature only)
  
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  A few Notes:

  -- Definitions:
  PvM: Player vs. Monster
  PvP: Player vs. Player
  Mob(s): Shorthand for monster. Origin can be disputed.

  -- Avoid “base” stat buffs.
  They are limited in their usefulness, especially as most people tend to concentrate level up points on Attack and Damage.

  -- Skills for gains in exp and gold are very popular and rightly so.

  -- Sustain is an enhancement that will lengthen the time the buff stays active, up to doubling the length the time.
  If you are shopping for buffs to buy from another player, look at the amount of sustain that the seller has.