As an anti-pretender contingency

[Jazzepi]

If you get it high enough, the pretender will be attacked by epic horrors he can't defend against. Even the moderate level horrors have astral claw attacks that leave the victim more horror marked. While this isn't terribly useful if the pretender is bearing down on your capital, it's a great long term strategy. Immortal pretenders have no way to remove horror marks.

I've been told, and I think this is right, that mages target units with the most hitpoints when they're trying to pick "biggest threats" for single target spells like paralyze. So giant pretenders should be easier to single out than ones like enchantress.

Jazzepi

[Valandil]

Generally, yes.
Horror marks are rather brutal long-term, but there is a good deal of luck involved.

Astral geyser, maybe? Even more marks, and pretty likely to hit if there is a small enemy force only.