Spell 'Evlin' ?

[Esteral]

Spell works wonders on bigger stacks.
Specially (ancient) vampires and Black Knights.

Good Black knight tactic is, use Teleport to get the knight across the board.
Then (if possible) put stone skin / magic shield (from archmage) and magic springs. After some troops are dead cast eviln on the dead stack and move the dead knight onto the space. You can also use the archmage ability to move the knight 1 space. This way the knight will not lose its action points.