Honestly, Final Fantasy as a series already has a solution to the self-sustain issue for DRK.

Blood Weapon

Blood Weapon (ブラッドウェポン, Buraddo Wepon?), also known as Sanguine Sword, is a recurring ability in the Final Fantasy series. Often a form of Limit Break, it grants the user the ability to drain an opponent's HP to heal themselves when attacking, similar to the Blood Sword and other blood equipment weapons.