Are you sure it is a quest given by the dark elf? I heard that it was from Vingalmo.Its a radiant quest from the dark elf in the castle hell eventually tell you to turn your spouse only works if their nords or imperials thought
Skyrim:The Gift - UESPWiki
Here is a link to the USEP.net Wiki that explains the quest, who gives it, and who you cannot turn.
That's why I love the Better Vampires Mod on PC. I can turn anyone in Skyrim into a vampire. So far, everyone in Whiterun are vampires (except the kids). Riverwood is next.Thank you for the link. I looked at it and it is pretty disheartening. Sondas Drenim and Farkas are the only suitable spouses out there. I am disappointed that the creators would make so many characters unable to be turned. It seems just as ridiculous as a player who chooses to be a vampire being unable to obtain the vampire hood. Argh!