Eeyoo Sekedoo Aat (Location)
The entrance to this special tomb is through the tablet room in the Tomb of Smenkhkare, in Uab Nome, which you'll pass through during the Seven Farmers quest.
Walk through the crack in the tablet room there to reach this place
Venture forward, continually scanning for stray silica as you go. You'll reach a platform that extend out over a large drop- there is enough water below for a leap of faith.
Proceed forward into the monolith room.
Touching the monolith activates it- you mau even do so accidentally while picking up silica.
Stick around for an atemporal message concerning Desmond and the catastrophe, but there's precious little new lore to glean here.