The Queen

The Queen. That's what they called her.
Few have ever seen a Night Fury, but fewer have seen the last Night Furies in the world. One of them, believed by humans to be the last one, resides with the Vikings. The others' locations are unknown.
Except for one.
Yin, known as The Last Shadow to her enemies, is the declared heir of the Red Death. She harbored a particular hatred for the humans on Berk, because they killed her soulmate. Or so she believed...
Years later, when she is the Queen, she comes across another Night Fury, carrying a human on his back. It turns out to be her supposedly dead soulmate, and with a new name as well.
Toothless.