When activated by a Dark Knight, each successful attack that lands (even for zero damage) will cause the user to lose 10% of his/her current HP and converts that amount directly into damage. This additional damage is added to the damage of the hit, not separately as with En-Spells.
When activated by a character with Dark Knight Support Job, each successful attack that lands will cause the user to lose 10% of his/her current HP and converts half that amount directly into damage.
Quick testing shows that the Souleater bonus from an augmented Dweomer Scythe may stack ontop of the previous pieces. See discussion.
The damage penalty of Souleater is not affected by Defense or VIT, Damage Taken+/-% such as Defending Ring(-10%) or Black Cuisses(+13%), the Red Mage spells Phalanx or Phalanx II, -% Physical Damage Taken gear such as Earth Staff/Darksteel armor, -% Magic Damage Taken gear such as Coral Scale Mail, nor is it affected by the Paladin ability Sentinel.
The damage penalty of Souleater can be nullified by Stoneskin for an amount equal to the HP equivalent that the Stoneskin provides.
Souleater is subject to physical damage modifiers, examples being open-mouthed Hpemde(+100% damage), and Slimes(-50% damage); additionally the modifiers for damage type (slashing, blunt, or piercing) based on the weapon used will affect the damage dealt by the ability--e.g., Skeletons take extra damage from Souleater when used with a Club.
While active, this ability increases the user's accuracy by 25.
HP converted into damage for a Weapon Skill is calculated for all hits using the same initial HP (i.e. A four hit weaponskill like Guillotine will inflict 40% of current HP and damage dealt, not each hit successively).
Each merit into Muted Soul will decrease the amount of enmity from, exclusively, Souleater damage by a -enmity value of 10.
When used in combination with Mana Wall the user will lose MP instead of HP.