Act 2 Quest 4: The Arcane Sanctuary
Quest Location: The Palace Cellar
Quest NPC: Drognan and Jerhyn
Quest Goal: Interact with Horazon's Journal
Flag: Allows you to enter the Palace Cellar & the Arcane Sanctuary + get the Waypoints
Accepting this quest flags you to receive the Palace Cellar Waypoint as well as the Arcane Sanctuary Waypoint. In order to complete this quest all you need to do is defeat The Summoner and inspect Horazon's Journal in the Arcane Sanctuary.
There are multiple other steps to this quest involving going through the Palace Cellar and discovering the Arcane Portal then taking it into the Arcane Sanctuary but they're all optional. Defeating The Summoner and inspecting Horazon's Journal are the only things you're actually required to do.
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.
Act 2 Quest 5: The Summoner will begin when you approach The Summoner in Arcane Sanctuary and will complete shortly after defeating him and traveling to Canyon of the Magi. Don't forget to speak with Atma after completing both of these quests for your flag.