Gate of Aaru (Aaru)
A stelae location immediately next to the entrance to Aaru. You actually may not pop this discovery on arrival, so check the northwest corner of your map for the precise location.
The stelae is hidden inside one of the overgrown hulls here- you'll have to pass through some hanging moss to reach it.
Read the stelae to get the Horizon Falls riddle component, and complete the location.