Drevin is a character in The Witcher's Side Effects premium module. He could be found stumbling around, drunk or hungover in the Temple Quarter. He had this story about a magic stone which was also a portal to untold riches. He also seemed to have a very interesting mysterious past. Is it true? Who knows, but it might be worth investigating.
- I am a mage hmm. Strike that... I was a mage and now am no more than a drunkard. And I could have been rich, powerful, a renegade...
- ...years ago I was a very promising student of magic, tremendous potential, truly tremendous... Yet fate played tricks and, uh, a certain propensity of mine prevented me from completing my internship and duly mastering the art.
After a particularly hard drinking episode started by a 30 
bet with the innkeeper, a jug of buttermilk is all he needs to tell Geralt all about the treasure and where to find this magic stone. It turns out, it is just in a nearby warehouse.
- ...the stone inside might interest you. It responds to spells, elemental ones to be sure. I wager if you cast a trick or two it'll take you to treasures beyond.
He then gives Geralt the key to said warehouse and wanders off.