Feature description

We have Giant Spawn, Husk Spawn, Skeleton Horse Spawn, Zombie Horse Spawn, and these have all been great, giving us mobs which were not so easy to get otherwise.
Now it is time to consider the Illusioner, another mob without a spawn egg and in need of an occasional appearance, at least until Mojang decides to add it somewhere.
I don't know what spawn rate would be great for it, nor where it would be best to appear (village raids?). Maybe more rare than witches?
The idea would be not to make the game too difficult, but to mix things up on occasion.

The problem I've noticed with the Illusioner is that the mob has been broken a bit. If you test it yourself by spawning it with the summon command, do you notice that too? Sometimes difficult to hit etc