[Radek]

In fact, it is not a quest. You need not get it from anybody. The only thing you need is the Rogue Stone in your inventory. Click the correct door.
But be careful. You cannot flee once you enter Twisted Rune. The only way out is through killing all monsters there. And the monsters are strong! A lich, an elder orb, a vampire draining 4 levels per hit, and a mage casting Time Stop and Gate to name some of them. Make a special save before you enter.