Changes

I think the item lore points in that direction. It uses the word "phantasm", it says the Umbaru priest went to Mbwiru Eikura and went back changed (Wikipedia says "after going through the portal for the first time, travelling to a destination unknown, he promptly returned, irrevocably changed and henceforth known as the Tall Man"), he had great power over the dead ("his main source of assistance comes in the form of corpses that he exhumes from the graveyards under his control; after digging up the cadavers, he crushes the body to the size of a dwarf, removes the brain and reanimates the body"), and that one finger was removed from him (if I remember well, such a thing happens in the first film).