Abyssal Drain
Level 56 | Ability | 60s Recast
Deal unaspected damage with a potency of 200 to your target and all enemies near it, and inflicts your primary target with Blood Price for 9 seconds.
Additional Effect: Restores HP
Cure Potency: 300
Blood Price: Heals the Dark Knight for each weaponskill or spell they land against the target.
Cure Potency: 300
At that point it'd be even as a burst heal at 4 targets, improved from 6, and over time even at a single target (300 direct + 3 hits' worth at 300 each for a total of 1200).
A second charge, since it'd actually include gameplay-affecting capacities, would also be appreciable as a trait.
Edit: You could also make it even more external-friendly, by having Blood Price cause the enemies' attacks to heal their victim for 300 potency each. (Imagine that before a Goobie Rush... It's just barely situational enough not to necessitate it's badly balanced down.)