Quell is a mage and he can be found in the Private Sensoriums in the Civic Festhall. He knows the key to Ravel's maze but will only reveal it for a price. That price is a piece of exotic candy (Quell has a serious sweet-tooth). Go to Vrischika's Curiosity Shoppe, and among the exotic items, you will find a Chocolate Quasit. Bring it to Quell and he'll tell you the key. From this point on, you can buy magical items from Quell. The candy he sells in his Candy Store are actually charms invoking spells ranging from Bless to Cloudkill. He also sells spells. Among the more impressive spells he sells are Power Word: Blind, Bladestorm and Chain Lightning Storm.
References[]
http://www.sorcerers.net/Games/Torment/Walkthrough/act5-findravel/findravel-festhall.php