After getting dark knight 60 and playing it for a while

[Reynhart]

Abyssal Drains heal can be relevant in multi-target fights. Unfortunately most fights prioritize single-target relevancy where there self healing is less potent.

So far, almost all fights in HW (I haven't done Alexander 2-4, yet) give us multiple targets so that we can use Abyssal Drain or Sole Survivor even during boss fights.
For example, the 1st miniboss in Alex 1 is far easier with Blood Price+Dark Arts+Abyssal Drain.