I'm trying to complete the Darkness Eternal quest given by Vicente Valtieri of the Dark Brotherhood. I've already completed the quest to cure my vampirism, and the last thing I've done with Darkness Eternal is visited Raminus Polus at the Arcane University.

I've read on UESP that the quest is complete when you kill Vicente, but in what context am I supposed to do this? Do I kill him as part of another quest, or should I do it by stealth in the DB Sanctuary? Should I leave him alive and leave the quest active in my journal, or what?

Thanks,
~UriahHeep

You will receive a quest in which to kill him. Just wait for that.

Killing Vincente Valtieri is not part of the quest to cure vampirism. It's part of a Dark Brotherhood quest, and it will be made very clear to you when it's time.

Since this is in C,H,&S, I'll tell you.

You kill the guy in a Dark Brotherhood quest called "The Purification".

The quest is completed becase Vicente is no longer around to give you a dose of his vampirisim. You'll have to get that somewhere else.

And the Cure for Vampirisim is not connected to this in any way. The only connection between the two is that you're killing a vampire, which is what Vicente is.