The island of Poko Kohara is the site of a legendary site: Engwithan ruins created by an offshoot of the doomed civilization drowned by Ondra.
Points of interest
You begin in the southwest corner. Follow the flags planted in the ground to a pillar covered in Engwithan runes ("Gods punish trespassers"). You can find two dried-up skeletons near it - definitely not the VTC you're looking for. Their camp is in the northwestern corner. Note that crossing the space between the two ruined towers will lead to an encounter with a swarm of sand grubs in quicksand which will slow you down (applying Slog Zone) and can randomly knock over.
The camp is abandoned and most of the supplies rotted through. You can interact with the lingering soul for a quick cutscene. The nearby well can be interacted with as part a scripted interaction. A rope was dropped down the ancient well, with nothing attached to it and water on the bottom. You can send someone down the well. A sack of gems is hidden just above the water. Athletics 4 allows you to swing towards it and recover it, while Dexterity 12 allows you to recover all the gems intact (8x Aquamarine, 3x Garnet, and 1x Emerald).
There is a small group of sand blights to the northeast, near a hidden stash containing a few gems. A more interesting spot is to the southeast, with a (large) xaurip camp that has no less than three priests and several champions and skirmishers. The prize is mostly supplies looted from the VTC expedition, and the unique pistol Thundercrack Pistol in a trapped crate.
Approaching the facade of the ruin will trigger another cutscene where Gian, Falero, Olara, and Beza are fleeing something that makes a tremendous amount of noise. Gian's sand-covered body is ripe for the picking, but offers only minor loot. Once you do approach the facade, you will learn just who did him in: An Engwithan Titan. It's a particularly challenging fight, especially since sand blights will join the fray once you knock him down to 50% strength.