I'm trying to do better about posting fannish stuff here, so here is my answer!
I don’t think the Everlasting Darkness is a thing. I think Maedhros goes to Mandos (if he chooses) or remains a spirit in Middle-earth (if he chooses not to go).
(Personally I think that he–they all–choose to go to Mandos.)
( Read more... )His seven sons leaped straightway to his side and took the selfsame vow together, each with drawn sword. They swore an oath which none shall break, and none should take, by the name of the Allfather, calling the Everlasting Dark upon them, if they kept it not; and Manwe they named in witness, and Varda, and the Holy Mount, vowing to pursue with vengeance and hatred to the ends of the world Vala, Demon, Elf, or Man as yet unborn, or any creature great or small, good or evil, that time should bring forth unto the end of days, whoso should hold or take or keep a Silmaril from their possession.