What creatures should populate the Norse Dwarven realm of Nidavellir?

[ksbsnowowl]

My 18th level PC's are going to be tromping through "the Underdark" of sorts, as they explore the Dwarven Realm of Nidavellir (note, this is a home-made cosmology of the Norse Nine Worlds; Nidavellir is in effect its own Material Plane with the three standard transitive planes).

I haven't gone digging exhaustively through the Icelandic Sagas for this kind of info, so I guess I'm looking for some ideas of "appropriate" creatures to populate this underground world, aside from Midgard Dwarves (FrB), and perhaps a run-in with some "half-dwarves" (normal D&D dwarves).  I need some "random" encounters as they are hoofing it through the tunnels.

There is also a decent chance of them using Shadow Walk to walk more quickly through the tunnels; some Shadow Creature encounters would also be good to have prepared.  It would generally just be Shadow Creature or Dark Creature versions of the typical Nidavellir creatures.

Oozes would be a good option, as well as Shriekers and Green Slime, but these are still far below my PC's level.

Maybe the Rothe from FR.  There would need to be some form of meat that the dwarves would hunt or farm, so this is an easy answer, I guess.

Earth and Magma elementals would be interesting.

Maur Giants (from Underdark) would be a decent option.

Certainly "trolls" of one form or another.  Maybe Fomorians.

Any other ideas?  Mind Flayers and stuff like that would be inappropriate to this setting, so many of the fall-back typical aberration monsters don't work well for my purposes.

[nijineko]

mephits? possibly refluffed a bit for setting?

[Nunkuruji]

If they are still master craftsmen, artificers, etc. to the level of creating well known artifacts

Of dwarven origin perhaps: Golems, Constructs, Living Spells

Of native critter variety, perhaps anything out of Underdark/Drow of the Underdark
Tunnel Worms, Disintegrators <3

[ksbsnowowl]

Of native critter variety, perhaps anything out of Underdark/Drow of the Underdark
Tunnel Worms, Disintegrators <3

What is a Disintegrator?  Did you mean an Annihilator?  I could maybe see turning it into a Gelatinous Creature as a special kind of Ooze. Wait, crap, Gelatinous Creatures are Aberrations, not Oozes... hmm...  Maybe an Earth Element Creature.
I already have planned an Earth Element Disenchanter to show up somewhere as a weird kind of elemental.

Another creature type I thought of for a random encounter was a Keening Spirit (special kind of Ghost) made from a Svartalf (ie - a Drow) that was killed after accidentally traveling to Nidavellir.  Or a Banshee; basically the same thing.

[Nunkuruji]

That's it, annihilator, it does disintegration

Familiar with Keening Spirit, when my players went through CotSQ they kind of didn't give a crap about it.
Heroes Feast puts down the fear aura.
Keening/WoB is a mix. The PBAoE is inferior to default WoB range option (grab Widen Su from ToM), but as an Su ignoring SR is nice.
Make sure to put some HD behind it for the Save, I guess.
Beyond that, basically just a ghost sorcerer, perhaps annoying depending on prep.

There wasn't much on page 1-2 of google for inspiration regarding Nid

[ksbsnowowl]

Constructs just never struck me as something that should be in a Norse-themed game.  This is perhaps a little wrong-headed, as the dwarves made a golden boar for... Freyr, that automatically resurrected itself each night after it was killed and feasted upon.  Regardless, the connotation that constructs bring just doesn't jive for me.

So I was scrolling through some monster manuals looking for ideas, and Bestiary of Krynn has the Proto Creature template.  This is exactly the kind of thing I need to fill this role.  What the dwarves have created is not magical robots, but rather magically-created life forms.  I know it is a slim line of distinction, but one works for me, and the other doesn't.  A proto creature advanced 30 HD Razor Boar... that I can get behind.  Now to play with other creature ideas.

Also, Century Worms (FF) and Razhak (BoK) would be decent options.