First, talk to Jerhyn in Lut Gholein. Enter the Arcane Sanctuary underneath his palace. Go down to Level 2 of The Harem and descend all three levels of The Palace Cellars (Waypoint on Level 1). You'll reach the Arcane Sanctuary (Waypoint) where you'll find Horazon's Journal. A portal will open to the Canyon of Magi (Waypoint) where Tal Rasha's Tomb is located. There are multiple entrances.