Absorbing creatures worked fine early on with my first creation. Shorty after however (still in the Southforge Citadel) the thad I summoned became invulnerable and couldn't be absorbed, but the fyora I had could be absorbed. A couple of maps later the fyora became invulnerable as well. I couldn't get them killed in battle either. So it seems that something gets corrupted in the save repeatedly.
I also started a new game with a warrior only character (no summoned critters) and I'm finding that mobs on the map occasionally go invulnerable. I have to leave the map and come back for them to reset to vulnerable so I can kill them.
I tried uninstalling-downloading-reinstalling the game, but it didn't help. The warrior game was started after the reinstall, and it has the invulnerable critters on map problem.
I'm running XP with latest patches, if that makes a difference.