On this page of the guide to Diablo 2 Resurrected you will learn how to start and complete The Summoner task.
The Summoner quest in Diablo 2 Resurrected begins after killing the titular Summoner in the Arcane Sanctuary (he's the mini-boss of the area).
This quest feels a bit artificial, as all you need to do to finish it is talking to Deckard Cain. However, a portal leading to the Canyon of the Magi where the Tal Rasha's Tomb is located can be activated here. To activate the portal, read the journal. Go through the portal, then return to town (you can do so by going through the waypoint) to immediately finish two conversations with Cain and two quests.