This mystery can be found in the southeast of Suthsexe, about 270m south of Crawleah, AC Valhalla.
As with any altar mystery, you’ll need to have a specific amount of a particular ingredient to offer before you can complete the mystery. In this case, it’s 50 leather. Leather is one of the most abundant materials in the game, so you should have plenty of it already. In the unlikely circumstance that you don’t, you’ll find it in abundance in minor treasure chests, and from pretty much any mammal that you manage to kill and loot.
Once you have all 50 leather, offer it at the altar to initiate a brief cutscene and complete the mystery.