Ud was once a man, but then he died and then tumbled down the lower dead realms called the Way Down, until he ended up as a skeleton creature in Fourth World, three layers beneath ours. He became a wight (skeleton) slave to the ruling elite there, known to Ud as the Ereshkagali, from his name for the god of the underworld when he was a human in the far past.
Over the course of millennia, he achieved what no one else had ever done, climbing from the bone dust lands of Fourth World, up to the Ghast swamps of Third World, through the Ghoul forests of Second World, and finally up to First World, the world of mortals.
And, after his initial elation at being able to feel as he once did the wates and clean air, free of rot and parasites the size of dogs passed, he began to realize a few truths.
-His mind was now in four parts, one for Ud the Wight (skeleton), Ud the Ghast, Ud the Ghoul and the newly minted Ud the Mortal.
-Fragmented memories meant that he only remembered pieces of his selves at a time.
-One of those fragments of memory brings with it a warning that all mortals should fear regarding the future of the Way Down.
Note-It's worth pointing out that the words and backstory of Ud when he was alive are related to a people that really existed, as well as a real language.