Damage enemies and drain more HP when user's HP is low.
Lifebreak, also known as Vengeance and Revenge, is an ability that deals damage equal to the difference between the user's maximum HP and current HP.
Attack with a wave of lost life force. The lower the user's HP, the greater damage inflicted.
Byblos's Vengeance acts similar to the Lifebreak spell, dealing damage equal to the difference in the user's Max HP and current HP. Byblos already knows this ability when he joins the party.
Deals damage equal to amount of HP lost.
Templars can use Lifebreak learned for 300 AP from the Kain's Lance, and deals damage equal to the HP lost. Curiously enough, the ability is Dark elemental, and thus the damage will be reversed on undead enemies. The range of the ability is equal to the weapon currently equipped.
Snipers can use Doom Archer. However, Doom Archer's damage is split between HP and MP.
Templars can use Lifebreak that is now Dark-elemental. Master Monks can use a ranged version called Withering Strike. Tricksters Agitate and Heritor's Viola are area-of-effect versions of Lifebreak. Snipers can use Marksman's Spite. Marksman's Spite damage is split between HP and MP.
Salve-Makers can create the Dark Breath effect using Compounding.