How do I start a quest involving that bastard? It seems like it would be fun...

His quest should start right after you find a new steward for Neloth.

What exactly is his quest?
=/

I just paid him.

He came up to me and told me that the person I made Neloth's new steward owed him 1000 gold, and that now that the steward is out of his reach, he expected me to pay him. I told him "or I could just kill you" and he walked away. Later I ran into three hired thugs who attacked me, but they were push overs, one ice storm spell killed all three in one shot, on adept. My character is a 20th level mage, he's not all that powerful. That's as far as I've gotten.

I waited until he went into the corner club then cast fury on him. The entire town killed him for me. The best part was when his own bodyguard got in the final blow.

I went to the steward and told him about Mogrul, but I think I failed a persuasion check. He basically told me that Mogrul's my problem now. Hmm, I DO have the frenzy spell...

Lol im gonna let these collectors face off against cicero, both DBI, frea, and 2 dremora lords

Well, that didn't go well at all. I went into sneak and cast the spell on Mogrul (and got a bounty). Ran downstairs as Slitter, Mogrul and Teldryn duked it out, Mogrul died and I cast calm on the other two. Walked outside and got pounced on by the guards. It wasn't pretty. I think my mage's next perk is silent casting.