Investigate the Tomb to find the Source of the Stink
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All Assassin’s Creed: Origins Guides

To explore the pyramid in search of Khaliset in The Hyena in Assassin’s Creed: Origins follow the tunnel and turn right and go through the crack in the wall:

Keep following the tunnel until you reach a hole in the ground. Drop into the hole and follow the tunnel until you find Khaliset.