Spell Plague

Cosmic event caused when Cyric, the Mad God, murdered Mystra, the goddess which maintained The Weave. With her death, magic destabilized and anything tinged with magic, to include artifacts, was affected.

This destabilizing effect caused the Nemesis to alter it's form in order to adapt. It expelled the souls of the legendary party, keeping only Arkin and Durell within. The Nemesis adopted a living vessel, and as Arkin's soul was the most powerful influence, the newly created being took on the appearance and memories of the real Arkin. Meanwhile, Eldar's avatar, who was on Faerun attempting to locate the Sword of Destiny, was also influenced by the plague, and became unable to rematerialize into his deific form.