I'm thinking he should initially be discovered between two colossal pillars in an underground tomb, with each pillar being inscribed with millions of words in a lost language (Daevite). Seth's body should be discovered within a sepulcher, anomalously well-preserved and clutching a vial of oil from the Tree of Life (in an interview he will explain that he and his mother once went to the Gates of Eden to ask for it. Though the Archangel Michael denied them at the time, Seth foresaw that he would eventually relent and give him the oil to ensure his resurrection). The body is somehow exposed to this oil (since the Foundation wouldn't do this, I'm thinking a GoI, maybe Sarkists, will be responsible), which resurrects him.
Seth will have an encyclopedic knowledge of the lost civilizations he witnessed during his original thousand year lifespan, as well as possess highly accurate precognition. The Foundation will find both these traits extremely useful in avoiding XK scenarios. Due to his clairvoyance he is aware that they're holding his brothers, and wishes to see them. It's obviously too risky for him to see Able, but since Cain is their living backup database he would have to know about Seth anyway, and they may decide to allow those two to have contact to keep them happy. I'm also planning to work in some existential/body horror.
