The most useful monster.

I mentioned Beholders in this thread. So, no need to repeat it. 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 has a slight advantage if it has "Forgetfulness," and Chaos with its Inferno or Fireball.
Life can only deal with some of them with a targeted Exorcism (what level is it?).
Nature... a swarm of wasps can be used. Or (just like Life, though) it can stabilize itself with its blessings.