Questions about the gameplay of Heroes 6.

I saw this worm in battle, but it moved too fast, and I didn't have time to notice who it was swallowing and how. How dangerous and effective are these worms? Or is it just a distraction?

I've only played with it a couple of times myself, but as far as I understand, as long as you don't kill it, it will slowly eat a unit of monsters on each turn. Accordingly, the unit being eaten skips its turn. As for whether you can choose the target for the worm, I don't remember. And it's not difficult to kill it (although it will take a certain number of turns).