DRK: What if for HP you get-

[Synestr]

FFIII had Souleater as the "ability" the job used. Consumed HP to perform medium to high DMG to all targets.

Also, from FFWIKIA: The Dark Knight is characterized by wearing tight, sometimes enclosed black armor and wielding a sword steeped in wicked powers. Their physical stats, with exception of Defense, are usually high, but their other stats depend on the game they appear in. The typical Dark Knight ability is Souleater, also known as Darkness, which allows them to sacrifice their HP to perform stronger attacks.

Also there is stated that the job was there from FFII, FFIII, FFIV, X-2 etc, of which darkness or souleater were used.

So unfortunately it was not an XI exclusive.