Act 2 Quest 5: The Summoner - Diablo 2
Quest Location: The Arcane Sanctuary
Quest NPC: Atma
Quest Goal: Defeat The Summoner
Flag: Canyon of the Magi & Tomb of Tal Rasha Entry
This quest becomes active when you get in range of The Summoner in the Arcane Sanctuary. You won't be able to access this zone at all until you first get The Arcane Sanctuary quest. Once you have this quest you can access both the Palace Cellar and Arcane Sanctuary and acquire both Waypoints as well.
All you need to do for this quest is defeat The Summoner and then return to Lut Gholein and speak with Atma who is right near where you portal in. Speaking with Atma completes the quest. In the same area as The Summoner you'll find Horazon's Journal which you'll need to inspect in order to complete The Arcane Sanctuary quest.
Note: You or someone else who is on the quest is required to click on Horazon's Journal; otherwise it will bug out and tell you that the quest can not be completed in this game. If this happens to you, make a new game and redo the quest and it'll complete normally.
Completing The Summoner quest will flag you to enter Canyon of the Magi and pick up the Waypoint in the zone. You'll also now be able to take Town Portals that are in Canyon of the Magi or Tomb of Tal Rasha. Sometimes the flag doesn't "kick in" right away and you have to log out your character then back in. Typically when this happens you have to speak with Atma again too in order to complete the quest a second time.