Arcane Sanctuary is the fourth quest in Act 2 of Diablo 2 Resurrected, found in Lut Gholain. It begins after finishing the Tainted Sun quest and talking to Drognan.
After talking to Drognan go find Jerhyn near the palace. Then enter the Harem. It has two floors, just like the Palace Cellar on the next two floors.
On the second floor of the Palace Cellar you'll find a portal. Activate it and go through. Before entering this new area you should make sure you resist fire/electricity. You'll appear in the Arcane Sanctuary. It's a large maze with many narrow corridors and portals leading to different platforms.
At the maze's end, you'll find the Summoner's lair. He's the mini-boss of the area who guards the portal leading to the next location. You need to eliminate him - make sure you've killed other monsters beforehand. Read the journal - you'll gain access to the Canyon of the Magi. Activate the waypoint and return to Deckard Cain to finish the task - at this moment, the Seven Tombs quest activates.