Ugly And Feared.

"My days may be numbered." Ragnarok thought to himself.

He is very clear about his precarious situation. On one side are wood elves that want to kill him and the other side is a terrifying Vampire Primogenitor that definitely has some unknown plans for him due to the fact that she is literally the mouthpiece of a demon god. Even if the demon god is taken out of the equation, Emperiti is more than enough to make him consider death at the hands of the wood elves. 

If the Emperiti Primogenitor were not so terrifying, she alone wouldn't be able to hold back an entire army of wood elves and continue to guard the devil's pit on her own for so long despite numerous attempts by the wood elves to destroy the crater. 

Vampires are feared for their ability to increase their numbers very quickly. From the little he has learned about her from the demon god, he knows that the Emperiti Primogenitor embodies that fear.