Uninvited guests.

I played on Veteran difficulty as a Warlock. Fire Walls couldn't be cast in the Castle. The Overlord somehow resurrected more of his creatures than he should have (turned 5 deaths into 10). And a Demon hero with 12+11 Archdevils, Demons with magic reflection – that's just ridiculous. Demonesses are stealing my Hydras (which counterattack adjacent enemy creatures when attacked – "what the...?").