South of the desert temple is a cave entrance. It can be hard to see, but it's there. This cave leads to a tunnel below the mountains, bringing you back to the lands surrounding the royal capital city. To the south east of the city, there are some purple flames. This is the dark presence. Speaking to Damia via the altar tells you the next steps.

However, this quest is never updated in the quest log. Instead, go to a teleport stone and ask Damia for quest help for what to do next. The recommended order of locations is as follows: the sewers of the capital, the temple in the woods, the desert pyramid, the cowgirl ranch (optional), the witches academy, the amazon island (not in the game yet).

This quest can't be completed in this version of the game (v0.351) since the last part is still in development.