A decently prepared thief can also park it in a city until he's ready and then duck out in a direction the duergar doesn't expect, double back and attack, or blackjack and gank him anyway right in front of the tribs with a horde of maran without even binding him.

Ravon isn't in Empire, he's in Scion and afaik all the duergar in Scion deleted. If a Fortress binder or two popped up, a Scion assassin or a couple duergar would be rolled.

Honestly, detect hidden just makes things harder, not impossible. It's the same with skills, bindings and blackjack failing.

But anyway I'm only rambling and applying the rules of Entropy Lich to Ravon.

Duergar don't scare my thug thieves anymore since they nerfed axe and mace.