Hall of the Mountain King
Like the City of Heroes, the Hall of the Mountain King is a mythical place in the afterlife where it is believed by some Erfworlders that rulers (and possibly only Royals) go after they croak to be judged. The sole reference to the Hall of the Mountain King so far has come from King Slately of Jetstone as he contemplated the end of his reign at the hands of Wanda and the decrypted armies of Gobwin Knob.LIAB Text 9
While the City of Heroes seems to be the afterlife destination for most warrior units and a widely held belief, the context in which the Hall of the Mountain King is referenced would indicate that one must, at least, be a ruler and seems to imply the one must be Royal to enter. It is also intimated, though not explicitly stated, that it is the Titans that judge those that enter the hall. However, there has only been one reference to this mythical place, from King Slately at a time of great duress. Given his Royalist fervor, our understanding of the implications of this myth are one sided at best and there are likely other interpretations of the Hall in Erfworld.
It is possible that the Hall of the Mountain King is a location within the City of Heroes.
Real World References
The song "In The Hall of the Mountain King" was written in the nineteenth century by Edvard Grieg for Henrik Ibsen's musical play Peer Gynt. The song was also covered by The Who in the extended version of the album The Who Sell Out.