The master thief is a mob added to the labyrinth in the most recent update. His behaviour is quite cruel and punishes incautious players, making it one of my favourite mobs.
He doesn't hit as hard as the minotaur. Instead his difficulty lies in high life skill, speed, and his ability to steal items. He wonders the labyrinth, I heard the minotaur also wonders around now, so you could bump into either while lost in there.
1) He seems to steal anything in the inventory except bags. This includes guzzling your health potions.
2) He's naturally very fast, and can probably go even faster if he steals a speed potion.
3) If you're on your own then he's been known to run away into the darkness after stealing your items.
4) He also pickpockets gold.
Vanner (one of the strongest top players) does not suggest trying to solo him. I think that is good advice.