Mastengwe is an Altmer residing in a ship called The Prowler, in the region of Khenarthi's Roost. She can be found sitting on the floor on the ship during the quest Cast Adrift and will inform the Vestige about a Maormer ritual of creating a storm.
Sergeant Firion asks the Vestige to find missing survivors in the Aldmeri Dominion shipwreck in the Shattered Shoals. Mastengwe can be found at The Prowler; she is a survivor of a storm-creating ritual performed by the Maormer.
"Those filthy Maormer dragged my mates up and tied them to these serpent-shaped statues. Then they started chanting, and lightning surrounded them! That's when the storm began to form around the island."
The Sea Vipers created the storm? "Yes, I'm sure of it. It was a terrible sight. The Sea Vipers were so caught up in the ritual, I slipped free. I tried to grab Suhr, but the lightning held him fast. Then one of the Sea Vipers yanked at him. I saw the Maormer's wrist glow."
- Why would the Sea Viper's wrist glow? "The Maormer wore lodestones on their wrists. It let them ignore those lightning bonds. But I couldn't find any more, so I jumped off the ship and swam for it. Please, don't let them sacrifice my crewmates!"