In this trip rudeus is taking, he will go into some sort of dungeon or something, there
His father, or paul. Will die and, Rudeus will lost one of his arms, i forgot which arm
(Correct me if im wrong)
(Also, i will not include all details)

So basically, Zeta is willingly gonna revive the uh 'diabolos thing that the diabolos cult tryna revive' and give shadow the eternal life, but actually shadow can just extend his own lifespan by himself.