Scabban is in the Asylum in the Crypt. The first quest of the Mages Guild in Valeia wants you to speak with him. In fact, you need not speak with him and kill him instead
There are certain problems with the Scabban quest. You'd better get the quest first by all your members of the Mages Guild of your party and then go to Asylum and speak (or "speak") with Scabban. This works. If you reach Scabban first and then you get the quest and return to Scabban then the questgiver need not detect that you have finished the quest and you have to repeat it.