[HoneyBadger]

I was thinking that if you just want interesting chaff, it might be fun to have thralls that, after a certain amount of damage has been done to them, a bat (with slight fear effect) explodes from their chests, where their heart used to be, and attacks.

It would still be a weak unit as a thrall with a pitchfork, and the bat would have like 1 hp and very low stats, but it would be thematically pretty spectacular to see a bloody bat exploding from a thrall's chest-cavity, it my opinion. You could have a somewhat tougher version that metamorphosed into a giant rat or even a wolf or a spider, or one with giant leeches bursting out of the still-living thrall to attack you, or even a high-level one possessed by a nasty demon. Maybe they could be national summons? It would be quite a surprise to attack a group of thralls, expecting to roll right over them, but then suddenly some of them are turning into monsters while others have monsters eating or exploding their way out of their own bodies to attack you.

That would make your basic thrall a really useful, still very weak unit, because it would serve as a camoflage for the more dangerous versions-you'd never know what you might get when you attack that group of thralls.