Let me see if i understand correctly: so in order to become a Nightingale, I must swear my afterlife to Nocturnal as well as my life? Does this mean that I give up Sovngarde? Because if so, then forget it, lady. When I completed the main quest my greatest joy was having the Nord heroes tell me they'd be waiting for me with a big feast when my time came. Nocturnal is only offering me a bit of luck in my thieving exploits, and I don't even get to keep the skeleton key. Blargh.

Or am I not understanding this correctly?

Yep you are understanding correctly.

However the service to Nocturnal might only be temporary...

You get really cool armour that helps you get lots of ladies so its worth it.

As Dragonborn , Akatosh has fist dibs on your soul. Any promise to Nocturnal is kind of pointless.

Akatosh is your father and the one who made your soul so if they want it they gotta go through him plus the fact at the end of the main story line spoiler ahead the guy who gaurds Sovngarde says you will have a place in sovngarde after your death. plus the fact that after you beat everything about 5 people not including akatosh think they will own your soul so they might have a big fight over it.

Akatosh vs. Nocturnal?
Would pay good money to see that!!

You get armour that reminds me of Batman, I say go for it.

Sell your soul to everyone who asks.

Run away with their money. And their pants.

There will probably be a loophole.

The Nightingale Armour alone is worth it already.

Doesn't Akatosh have dibs on your soul anyway, since being Dovahkiin was his gift?

Shor and Tsun seem to have dibs on your soul as well, at least that's the impression I got during the MQ.

Is there anything in game saying Akatosh is getting your soul? I don't remember seeing anything.

But Hercine an Nocturnal can fight over it.

So it's either a huge endless feast and mead listening to warriors tell their tales for centuries to come. Or, you guard a damp, dark, and scarcely populated cave for all eternity while feasting on uh rations? Oh not to mention there's the chance someone steals the skeleton key in the future and just like before all guardians go mad or become powerless. That's the possible fate awaiting you.

Nocturnal get owned outside her realm.And I'm not giving my Dragonborn soul to Hircine and neither to Nocturnal and neither going to Sovngard.I want live like Talos,as a Man-God.Stupid Deadras,fight the Dragonborn is the same as fight Alduin.

My Thieve's Guild character is a Breton. I doubt she'd end up in Sovngarde anyway.

My soul already "belongs" to fourteen other Daedric princes. What's one more?

I'm sure that The Divines are going claim the Dragonborn soul.if the dragonborn is pushed by Hircine,Nocturnal whatever Prince wants to claim his/her soul.The Divines are probaly showing up and kick them out of there.