It may be involved in an upcoming update, or its development and release may have been cancelled entirely.
Muspell is a realm that is represented by the M sigil on the World Gate. The World Gate itself describes Muspell as "A world engulfed in flames. It's kind of beautiful." Muspell was intended to the homeworld of the Auspah by Mod Rowley when the World Gate was designed for Fate of the Gods. However there are other potentials for the realm, such as it being "Zamorak's world", referenced in elf lore, or the location where Moia went with Zamorak as no official decision has been made.