The Elder Scrolls: It's the IX Divines, you milk-drinkers

[Mobius Twist]

Being able to come up to strength supposedly-possessed by Mercer Frey is the sales pitch that Karliah gives you. To Nocturnal in the midst of the actual ritual she gives a different sales pitch: Have two more souls to serve you in life and death and in return bring me back into your tender mercies. Even Nocturnal notes that the deal is heavily weighed in the daedric prince's favor.

At that point you're in the middle of the ceremony and pissing off a daedric prince in her own domain (the place her power is strongest in the mortal plane) is not a good idea whether you suddenly regret your choice or not.

You're sold a bad bill of goods, from what I understand, and have no choice but to go along with it once you figure that out.

I wouldn't be surprised if most Last Dragonborn players would then proceed to become vampires just to cheat all the princes that now own their soul.