|A hideous humanoid figure with metal skin. While it lacks magical attacks and is slow of foot, it makes up for these failings with brute strength.|
An iron devil is a tier-4 demon with decent AC and multiple resistances. It's as slow as a naga and it can't swim, so escaping isn't difficult. If you do choose to face them head on, they may be a threat to low-level characters, as they tend to wield heavy weapons. They normally appear in the Abyss, Pandemonium, and the Hells (especially the Iron City of Dis), but can also appear in vaults in the early Dungeon (safely locked away, of course).
Tips & Tricks
If you're in Hell, you can pretty much ignore these. If you worship the Shining One or Makhleb, kill them for the healing. If you do find one in the early dungeon due to a vault, you're unlikely to be able to kill it due to its AC and resistances, so just walk away.
Prior to 0.15, iron devils did not come armed, and were significantly less threatening.