Didn’t happen to me but the ‘preparing’ part seems to be an Umbra arthron - when killed it burrows on the ground and when someone gets close it prepares to come out of the ground on the next turn. Actually, it’s kind of a bigger shadow, not the original arthron. The bug may be related to it.
I had the exact same situation with the only umbra arthron I encountered thus far: it was the last enemy, but my game didn’t freeze.
Thanks for the report. We looked into it and managed to identify the problem. We will fix it in the next patch. Unfortunately, there is no workaround till then that I am aware of.
I only killed one umbra and the game didn’t hang, but as everyone here is getting hangs to be on the safe side I’ve been paralyzing “alive” umbras so that they don’t “prepare”. Just a workaround if anyone wants to try it.