What's the downside to taking the Nightingale oath? I am a bit confused because here's another daedric (?) power that has a claim on my soul. First it's Hircine, now it's Nocturnal. What about my dragonblood?

Will I never get to Sovengarde?

It's really complicated when it comes to dealing with what happens to the Dragonborn in his/her afterlife. I think it's up to the Dragonborn, but some have said nothing will happen, and that the Dragonborn will go no where in the afterlife and just be forgotten. I remembering reading that one from someone in the forums, but I am not entirely sure what happens. Maybe perhaps Daedric Lords will fight over your soul to see who gets it, or maybe since you have the soul of the Dragon, you may get to choose.

If you're a werewolf, according to Kodlak, your wolf spirit comes to claim you before you reach the Hall of Valor, which if you ask me, should be pretty easy as a dovahkiin to kill. As far as Nocturnal goes, if you speak to Tsun, he basically says you wont be getting in when you die due to your binding contract, if you tell him you are a nightingale. Between the three, it seems that Nocturnal wins your soul in the end. If it was me, I'd tell nocturnal "Thanks for the armor, but I want to keep my soul, thanks. You can have your stupid powers. I dont need them to kill that scrub."

Can you take Mercer down without joining the Nightingales? That's sure what I'd prefer but the dialog with Karliah doesn't seem to give you an option to say "no, I don't want to take the oath." heck, I didn't even want the dern Nightingale armour. It's grossly underpowered compared to the Ebony armour I have.

Maybe I'm missing something?

I've got a save just before taking the oath...

To complete the theives guilds quests you must become a Nightingale, just like how you need to become one of Hircene's buddies to complete the Companions. It's stupid, but what are you gonna do?

Temporarily, you can free yourself from your pact with Hircine.

The soul question is a hard one. If you do all the quests for the daedra lords there will be maybe 8 or so of them who have a claim to your soul, in addition to the fact that s does most likely Akatosh.
However, it is very hard to say what will happen. We know that a Dragonsoul is not the same as a normal soul, so it is entirely possible that Akatosh is the only one that can claim it because of his Dragon nature.
At the end of the day I have no idea.

Sadly no. You have to join the Nightingales. If you find the armor underpowered, it might be because its light armor. I honestly dont care about that though, I just think it looks badass.

Interesting. I know I can cure myself of Lycanthropy but that doesn't seem possible with Noctural's pact.

So...bottom line I either become a Nightingale or leave Mercer alive and head of the TG?

You either join the NG's kill him, or no quest progression. If this was real life, theres no way he'd lead the TG, since they all hate him now.

Akatosh trumps the Daedric Princes.

Thats what I'd like to think, but Nocturnal's got me worried with that pact thing, especially after what Tsun says about it.

He's just jealous because he's not a Dragonborn.

This is why I only do DB and TG on rogue style characters. I always make my Nord a werewolf, but I eventually cure it after I'm done with Dawnguard, and I always screw the Daedra Lords and do whats right if I can. Molag Bal's mission, I usually avoid on my "good" character. I basically avoid doing anything super evil that can land my soul anywhere but Sovngarde for roleplay reasons. Thats why I destroy the Dark Brotherhood. Thats also why it took me so long to get the oblivion walker achievement.

I don't care what Nocturnal does I am the damn Dragonborn! I do what I please! Daedric Lords screw people over all the time. It's time for the Dragonborn to troll them.