The being now known only as The Nameless One is one of the closest things the multiverse has to an immortal. Precisely how long he has lived or how many reincarnations he has had is unknown, but judging from some of the contacts he has made he is probably at least several thousand years old, and a former "incarnation" notes that he has died several thousand times.
His place of birth and early history have been lost, but at some point he committed a horrible crime. The precise nature of this crime is never expressed directly, though it was clearly severe enough to damn his soul even if he spent the rest of his life doing nothing but good. Hoping to avoid the horrors of an eternity in the Blood War, he sought out the night hag Ravel Puzzlewell and presented her with the challenge to make him immortal, planning to spend the rest of his existence trying to atone for what he had done. As payment, he presented her with the riddle: "What can change the nature of a man?" Judging from his current appearance, he was likely around thirty at the time. Ravel's immortality ritual succeeded, though, rather than granting him eternal life, Ravel simply stripped away his mortality (likely in addition to other enchantments that regenerate his body). To test her ritual, Ravel immediately killed The Nameless One, and, when he recovered, he had lost all of his memories. He no longer remembered the crime that caused him to seek immortality, or that he was immortal at all, or his own name.