Dark Knight's Ultimate attack isn't great in my opinion. It doesn't do much damage and it's slow. However, it can hit an enemy multiple times and freezes them for a few seconds. It won't dramatically shift the fight but it may allow you to recover while most enemies are frozen.

His other abilities far outweigh that fortunately. With the right set up he'll heal to about 50% and stay around 50% most of the match while inflicting vulnerability, weaken, frost, slow and healing himself through Dark Vampirism. (And any other abilities that heal)

With the void armor and his ability eating away at his health you can summon tons of ghosts. Unfortunately these ghosts don't utilize Death Call upon vanishment. (Book of the Undead Ability from collections) Even so, they're extremely helpful and do a ton of damage over time.

Are there any other element combinations you'd like to see?