Robe of Arcane Aptitude

Loot from Zhadroth, a lich you "pulled" into the plane of your existence during Zaviak's Vision Quest.

In-game description
"The provenance of this dark purple robe has been lost in the mists of history, but a faint odor of stale magic lingers in the air around the threadbare garment. The one who wore it was clearly no stranger to the Weave. Closely examining the robe, you are able to pick out a stylized flame design woven into the fabric. It seems likely the intricate pattern was always subtle, but even were that not true, countless years of wear have rendered the flames all but invisible."