What exactly are you supposed to steal from him to complete the Thief Guild quest? How high does a Guild Member's rank go? 9 right?

As I recall, you have to find him AND his treasure. He then gives you a something-or-
another for proof, and moves on to start again. I guess most people first meet him on
the eastern half of the Enchanted Isle - that's not where the treasure is. Don't kill him.

See that is the problem. I already found him and his treasure.

To solve the quest you have to go through the Temple of Isis/Anephas' Tomb, visit the area Kerah is in, swim to the surface and locate a chest - the item you need is in that chest.

Wyv

Yes, but I already did all that BEFORE I got the quest. Maybe I'll deep fry the Leprechaun, and return his burnt corpse to the little rat. "Here's my PROOF!"

Good luck trying to get him, he casts Vanish ... so it is a pain to get him even with high damage spells (my wife did it in 15 minutes though).

Guild membership rank stops at 7, there are more promotion quests, probably for those to catch up who missed out on an earlier one.

Ah, I should have known. My Paladin is in 2 guilds and both are at lvl 7. How foolish of me. Yes, when I find him where his treasure is, I could fry him .

NanoDjinn, be quick with the frying: he'll disappear within a few secs.
And there is no advantage extra at 7th guild level, so 6th is the highest useful one.