Temple of Arkay is a location added by Undeath
The temple is structured similar to old Nordic burial mounds. The entrance is hidden inside the overarching tomb, with a shrine of Arkay to be found within.
A group of priests and priestesses ran this temple to honor Arkay. Their work was cut short when Gundar and his necromancers attacked the temple and killed all. One of the priests was taken hostage so they could take his heart for use in their ritual.
Later, the Dragonborn came to kill the necromancers to find out what they were seeking. The Dragonborn may have chosen to rescue or kill the captive priest for his heart.