What if The Last Dragonborn was a twin, and his brother was roaming Skyrim. Eventually, the two will reunite when the time comes.

I, Kisvar, the only known Dragonborn, have defeated Alduin in Sovngarde. I joined the side of the Stormcloaks in the Civil War, and the Empire is subdued once and for all. Skyrim is at peace, and I am renowned across the Nine Holds. You might think my life has slowed down some now, right? That I'm taking a well-earned break from adventuring? .... No. I make my living as an adventurer, and in secret, as the Guildmaster of the Thieves Guild. I thrive on adventure and attention. You wouldn't take a break from breathing, would you? Didn't think so. Now I'm starting on a new quest, one that started with two strangely-dressed men and an order to kill me from some person named Miraak....

Ventus has been ignoring Vanitas for a long time and Vanitas is finally tired of it.