The most useful monster.

I mentioned Beholders in this thread. So, no need to make things up. I also said (and I still say) that it's a pity they can't be recruited in a castle. And they are rare even on the map. But you often have to fight them for resources and money. Here, Haven is more or less lucky if it has "Forgetfulness," and Chaos with its Inferno or Fireball.
Life can only deal with them in the early stages with a targeted Exorcism (what level is the general one?).
Nature... a swarm of wasps can be used. Or (just like Life, though) it can stabilize itself with its blessings.