Becoming Legend is a feature added in patch 2.2. A full guide blog on the feature is available here and is highly suggested. 

Essentially, becoming Legend allows you to start your game back over from the beginning. You'll start at level 1 with no skills, spells, or items. The only Weapons, Armor, or Accessories that you keep is the legend gear earned from Daily Dungeon. Epic coins and Key items stay, as do Achievements and Statistics.

There are two major benefits to becoming Legend:

  1. Legend Perks: You earn 3 perk points at level 60, and another 3 every 10 levels, up to 15 points at level 100. When you Legend the first time, those points are awarded to you and the first row (five total) of perks unlocks. These perks are permanent and persist each time you Legend. The first column costs 1 perk point per rank, the second costs 2, third 3, fourth 5, and the final column costs 10 points for each rank. They're in rough categories: the first is mostly main stats, the second one gives bonuses to weapon damage depending on type, the third is roughly skills that you can get with regular levels, the fourth is chance effects, and the final ones are unique bonuses not found anywhere else. Warning: Spending these perk points is currently permanent - there is no way to reset them and spend them elsewhere.
  2. Legend Bonuses: Each time you Legend, you get a Legend level. The first 10 of these (so far, more are in development) award either a couple of bonuses or a boost to experience gained. These bonuses are permanent and persist each time you Legend. For a list of these bonuses view the Legend Guide Blog.


Perks Table Edit

Category Main Stats Weapon Bonuses Skills Chance Unique
Point Cost 1 2 3 5 10
Legend 1 Max HP +5 Sword Damage +5% Magic Missile Count +1 Gain 10 HP on spellcast +2% Summon +10 meteors on revive
Legend 2 Max MP +5 Dagger and knife Crit Chance +5% Double Cast +1% Duplicate Items +1% Bomb Brigade on Block +1%
Legend 3 Defense +2.8% Axe and Hammer Crit Damage+20% Item Storage +3 Area Damage while Charging +10 HP below 10% - Level +1 Ghost Knight
Legend 4 Luck +1 Staff and Wand Melee Damage +5 Bonus Damage below 20% HP +5% One extra Revive +10% Summon Hydra on Fire spellcast +20%
Legend 5 Int +2 Boss Damage +5% Free Cast +2% Gain MP on item use +1% Max HP +20, Max MP +10, Damage +5

These amounts are per rank. Most skills have just 5 or 10 ranks, a few have 1 or even 40.