The Tarkenfaen (also spelled Tarkenfaeyn, pronounced Tark-en-fain) is an ancient, extra-planer artifact created by a long dead civilization in a far away and long forgotten world. It is seemingly indestructible and capable of raising the dead of any being (mortal or otherwise) and completely circumventing the will of even the gods.
The artifact- a horn in appearance- was created in a distant plane now long forgotten. The name Tarkenfaeyn is not its actual name (it is unclear if it actually has a real name at all), and is merely what most scholars refer to it as in Toril. Tarkenfaeyn is an elven word, roughly translating to "Life Giver".
Legend tells that Myrkul, then god of the dead, saw the artifact as blasphemous, and so sent his servant Medorion to retrieve it and slaughter the entire race for its transgression. The black angel accomplished this task without mercy, and by the end of it the whole civiliztions world lay dead by the dark being. Pleased with Medorion (and equally annoyed at his own inability to destroy the horn), Myrkul named him The Keeper of the Horn, and rewarded him with the Black Sun Citadel, a pocket plane which would become Medorions keep. However, Myrkul took measures to ensure that Medorion himself would never attempt to use the horn; should it ever be sounded, it would destroy the angels very essence.
If there is a way to destroy the horn, even the gods do not know of it. It is capable of resurrecting anything with no limitations by divine or even arcane law. Even should a soul have been completely disolved in the wall of the Faithless or held by a god, if the horn is used that thing would be brought back.