ToME: the Tales of Maj'Eyal

Not the Derth arena quest, but the actual arena module. I'm between wave 50 and 55. I've cleared out all the enemies in this wave but didn't get the notification that the wave is finished. I have tried aoe on every tile to see if something is hiding, I've waited for about 100 turns, but no cigar.

I suspect that the 'Shadow Simulacrum' Anorithil talent has something to do with it, since this is the first time I used it. I will try to test it more when I have time.