Originally Posted by American Maid
Surtur was trapped in Muspelheim by Odin and his brothers at the dawn of time. I have the impression that the astral realms are different from any of the Nine Realms, and Muspelheim is one of the Nine Realms.
That's how it's presented in the Simonson run, but I have the impression that's pretty standard for the various Marvel continuities.
I'm thinking of ways to tie things together, but it could be as simple as that
You know how there was a little bit of talk about Malekith having some sort of past?
And there's a story behind his face?
It's kind of like that...
In my scenario Surtur has already escaped once before and we learn about him/Muspellheim through flashbacks and what Malekith did in the past.
Which was try to start Ragnarok early with the whole twilight arc.
However, Odin put a stop to this.
As I keep trying to hint, the same things that happened in Malekith's past may be connected to what has trapped Surtur in his present state.
Odin didn't know Malekith was involved, and when he cast a spell to banish all involved to a new dimension in the astral side of existence the spell saw Malekith as only half-responsible. Odin was unaware, and only half of Malekith's existence was sealed away for all eternity. He now has a permanent connection to this astral plane/temporal vortex that Odin created, and Odin doesn't know (he assumes the spell must have taken anyone involved and sealed them away permanently). In my theory Ragnarok already began and Odin put a stop to it, we get to see some of it in flashbacks. But it didn't go down like the myths.
Malekith tried to start it early, so it's only half begun and has yet to play itself out fully.
It's almost like it's inevitable that he'll return and Malekith believes he has to finish starting Ragnarok, but it'll occur on its own so long as Malekith is still tied to physical reality as well. There's a catch, if they just kill him and free his spirit then Surtur is free because of Malekith's past connection to him and to the astral plane. Ragnarok is destined to go forward and there are a number of dark forces now pushing towards it.
That's the thing, Ragnarok started at some point prior to the first movie, and Odin dealt with it quietly. Surtur was released and sealed away somewhere Odin hoped the Dark elves couldn't free him from again. Malekith wants to complete what he started a long time ago.
I don't know exactly what Malekith did in the past, but part of his backstory explains his face and the twilight between the physical and non-physical that was created because of that incident.
Surtur's already faced off against Odin a long time ago and Malekith is seen as a traitor, although Malekith's direct involvement was not known and the dark elves themselves were branded as traitors.
In the past Malekith and the dark elves already tried to initiate Ragnarok by freeing Surtur but Odin was able to stop Surtur...
He actually defeated Surtur once before. Malekith escaped unseen, but not unscathed.
However, he couldn't actually kill Surtur or get him back to Muspelheim so he trapped him in the astral realms. Which, unfortunately, is one of the places the Other made his home, and the realm he controls and rules now...
To banish Surtur to the astral realms, and essentially erase his from existence and imprison his spirit, Odin had to make a pact with some of the other groups that upset the Vanir. He saw it as necessary at the time and some of the Vanir see things a different way.