Immortal Realms
by RipvanwormerImmortal realms from Warriors of Heaven: These are terrible, but they may be the closest thing to official names for the various Immortal realms. Apologies for this post, Havard, since I know it's not what you're looking for at all, but I started writing it and then I had invested too much time not to finish. Maybe you could steal some of the names from it and turn them into Outer Planes? Like, maybe Calitha Starbrow lives in an Outer Plane called Aquarianus.
Al'Kalim: Prime Material Plane (what? why?)
Alphatia: Mount Celestia/Mercuria/Goldfire (this is the realm of the Indian sun gods Surya and Mitra. Why would Alphatia be there?)
Asterius: Bytopia/Dothion/Golden Hills (this is the realm of the gnomish pantheon. Why would Asterius be there?)
Atruaghin: Arcadia/Abellio/Azcan Plateau (Atruaghin is worshiped by the Azcans [as Quetzalcoatl], but it would be strange for him to name his realm after them exclusively when he is also worshiped by the Atruaghin Clans and others)
Benekander: Prime Material Plane (every Immortal should have a domain in the Outer Planes, even Benekander)
Calitha Starbrow: Elysium/Thalasia/Aquarianus (this one is actually new, I think)
Diulanna: Elysium/Amoria/The City of the Star (this is Ishtar's realm, and doesn't seem terribly appropriate; she should probably be in the Beastlands instead)
Djaea: Elysium/Eronia/Great Mother's Garden (this realm is already shared by the Forgotten Realms goddess Chauntea and by the Chinese goddess Liu, so okay)
Eiryndul: Arborea/Olympus/Arvandor and Ysgard/Ysgard/Alfheim (this would make more sense if he were the same entity as the AD&D elven god Erevan Ilesere)
Faunus: Seelie Court (wanders) and Arborea/Olympus/Grandfather Oak (maybe if he's the same as the Seelie Court deity Damh)
Halav: Ysgard/Nidavellir/Hall of Arms (apparently a new realm)
Ilsundal: Arborea/Olympus/Arvandor (I'm not sure about just sticking him in with the Seldarine, unless he and Corellon Larethian are the same entity)
Ixion: Arborea/Amoria/Nest of the Phoenix (this is a literal nest made of twigs with a bird living in it and not a place for a major Immortal)
Ka: Beastlands/Brux/Library of All Knowledge (this is where the Forgotten Realms gods Deneir and Milil live) and Elysium/Eronia/Seeing Glade (apparently new)
Kagyar: Bytopia/Dothion/Mithral Forge (this is also the realm of the gnomish god Flandal Steelskin and the Japanese god Ama-Tsu-Mara)
Khoronus: Mechanus/Wheel of Time (this is the realm of the Greyhawk god Lendor)
Koryis: Mount Celestia/Lunia/Heart's Faith (this is the gate-town to the Outlands; what, does Koryis have a house there?)
Mealiden Starwatcher: Arborea/Olympus/Arvandor (just thrown in with the Seldarine again; but then again, where else would he be?)
Noumena: Arcadia/Buxenus/Heliopolis and Ysgard/Ysgard/Asgard (so he lives with both the Egyptian and Norse pantheons? Marco says he's a former Nithian pharoah and sponsored by Odin, so I guess that makes some sense)
Ordana: Arborea/Olympus/Arvandor (I guess my thing is that Arvandor was already thoroughly mapped in On Hallowed Ground and I'm not sure where all the Mystaran elven Immortals would fit. There are other obscure elven deities that On Hallowed Ground didn't mention, though)
Petra: Arcadia/Abellio/Sentinel Tower (apparently new?)
Pflarr: Beastlands/Brux/Temple of the Jackal (apparently new)
Protius: Elemental Water/Spuma (new)
Rad: Quasielemental Plane of Radiance/Pool of Luminescence (I actually like this)
Rafiel: Elysium/Amoria/Pax Benefice (this is a surface town ruled by a dwarf. Not appropriate for the patron of the shadow elves)
Rathanos: Ysgard/Muspelheim/Njarlok (I like putting things other than fire giants in Muspelheim, but this is the name of a town in Surtr's realm. Give him his own realm)
Razud: Arcadia/Abellio/Hall of the Granite Tree (new, I think)
Tarastia: Mount Celestia/Mertion/Arvenna (this is the home of the archons of war, and apparently also the Greyhawk goddess Mayaheine)
Terra: Arcadia/Abellio/Home (new?)
Valerias: Arborea/Olympus/Heartfire (this is actually Sune's quarter in the city of Brightwater; Valerias should probably get her own quarter unless she and Sune are one and the same)
Vanya: Ysgard/Ysgard/Heldannis (New?)
Zirchev: Beastlands/Krigala/Recluse (New?)
I suppose I should list the AD&D names for the realms of the other entities shared by Mystara and Planescape, though.
Bast/Bastet: Ysgard/Ysgard/Merratet
Demogorgon: Abyss/The Gaping Maw (or Abysm, though this refers to Demogorgon's fortress in 3e)
Forsetti/Forsetta: Ysgard/Ysgard/Asgard (his hall is Glitnir)
Frey: Ysgard/Ysgard/Vanaheim and Ysgard/Ysgard/Asgard
Freyja: Ysgard/Ysgard/Vanaheim and Ysgard/Ysgard/Asgard (her hall is called Sussrumnir)
Garl Glittergold/Garal Glitterlode: Bytopia/Dothion/The Golden Hills (which means he shares a realm with both Kagyar [since the Mithral Forge is part of this realm] and Asterius; Garl's portion of the realm is known as Glitterhome)
Hel: Gray Waste/Niflheim/Niflheim
Ilneval/Karaash: Acheron/Avalas/Nishrek
Kurtulmak/the Shining One: Baator/Avernus/Draukari
Loki: Pandemonium/Pandesmos/Winter's Hall and Ysgard/Ysgard/Asgard
Maglubiyet/Wogar: Acheron/Avalas/Clangor
Odin: Ysgard/Ysgard/Asgard (his hall is Gladsheim)
Orcus: Abyss/Thanatos (obviously this doesn't work for Mystara, since Thanatos is a prominent Immortal. In Pathfinder, Orcus's realm is called Uligor)
Skuld: Outlands/Well of Urd
Thor: Ysgard/Ysgard/Asgard (his hall is called Thrudheim)
Tyche: Arborea/Olympus/Olympus (as Tymora: Arborea/Olympus/Brightwater; her realm there is the Quarter of the Great Wheel)
Vaprak/Jammudaru: Abyss/Shatterstone
Yeenoghu/Ranivorus: Abyss/Yeenoghu's Realm (formerly the Savage Searing, before Yeenoghu took it over; also known as the Seeping Woods)