I mean it seems pretty common for mortals to dedicate themselves to more than one Daedra Lord, so who gets the soul when the mortal dies?

I have absolutely no idea.

In the Dragonborn's case, he gets absorbed back to Akatosh or Lorkhan/Talos I think. It's late and I'm shaky on the lore even when I'm not running entirely on caffiene.

The Dragonborn returns back to Akatosh once he/she passes away, regardless what they worship. If the Dragonborn doesn't return to Akatosh, then definitely Kynareth.

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Altmer - Aetherius

Argonian - The Hist

Bosmer - Y'ffre's Realm

Breton -

Dunmer - One of the Tribunal's Realms (Azura, Boethiah, or Mephala)

Imperial - Aetherius

Khajiit - Alsweyr and/or Azura's Realm

Nord - Sovngarde

Orc - Malacath's Realm

Redguard - Far Shores

Daedra Worshipers - The Daedric Lord's Realm

Were-creatures - Hircine's Hunting Grounds

Vampires - Most likely Coldharbor, since Molag Bal created the first vampire

M'aiq is still figuring out where is the Altmer's, Breton's, Khajiit's, and Redguard's afterlives.

Makes sense for the Dragonborn, kinda.. But the CoC's worship of multiple Daedric Princes makes no sense whatsoever.

M'aiq thinks the Champion of Cyrodiil goes to the realm of Skooma and Sweetrolls.

Altmer's got to aethuris (I think) and khajiit go to Alsweyr (Khajiit for where all things end) and/or moonshadow the realm of azura.These both being realms of moons and change,they can travel between them freely.

'scuse me?

M'aiq now needs the Bretons and Redguards.

M'aiq's realm is known as the Realm of Skooma and Sweetrolls. There are rolling hills made up of Sweetrolls and rivers, lakes, etc. made up of Skooma.

The Far shores for the Redguard and most likely Aetherius for the Bretons though that is debatable.

http://i.imgur.com/sQi4OiI.gif

M'aiq shall need the Bretons' afterlife.

Only those who worship M'aiq are allowed to enter M'aiq's realm.

The CoC mantles Sheogorath, so they'll be in the SI, ruling over it.

All worship m'aiq!The warrior cat monk of skyrim!

I don't think you go to a Daedric Prince's realm merely for worshiping them, I think it has to be something more. Becoming a werewolf or a vampire is enough to tie you to a Daedric Prince, but if you do a quick favor for Peryite for a nice shield your not going to his realm. If you become a Nightingale you must sign a contract with Nocturnal. When you sign a contract that's when you end up in a realm of Oblivion. Now for one multiple Princes do have a claim on your soul, for example a werewolf Nightingale, I like to think one of two things happen. Nocturnal gets full claim of your soul, because I think a contract has precedence over being a werewolf (Especially because lycanthropy can be cured even after death.) Or, a Persephone like thing happens. You spend sometime in the Evergloam and sometime in the Hunting Grounds.

Doing a task wouldn't get you there, but worshiping them would.

They roll dice. Dice made out of ground-up human bones, and with pips painted in arterial blood.

I guess whatever Prince has first dibs, then . If you worship Molag Bal and then start worshiping Mehrunes Dagon, Bal gets your soul.

They decide your character will be immortal and ageless because they don't want to go through the headache of trying to figure out who gets you.

As evidenced by the plight of the Companions, service to a daedra hijacks the racial afterlife.