You beat me to it lol.
@Sielgaudys - The OP is asking about something that is in game, the island located in the east part of the Highlands, and it’s called Telith’s Island, the location for the Sanctuary Ruins POI, Telith also been the name of the ghost. The ghost itself have been reported multiple times as bugged and not appearing at the location since late 2018.
Telith is the daughter of The Witch Queen, what Razma of Shem could see when she wore her mask. Telith and Tyros the Deathbringer were a couple and had a child named Xalthar together. Telith’s Island, Telith’s Lament and Telith’s Sorrow are dedicated to her. Tyros of course was the half breed child born of a human and a being of the Exiled Lands. He later killed the Priest King (his father) in a battle in the north.
With that said, The hidden achievements currently added but not achievable yet are for the new DLC map and not to be confused with what is been asked here, that’s Siptah Island?, not in yet, again, for more info, see below:
Siptah stuff (upcoming DLC Map):
Telith’s Island: