Fenris, steadfast and loyal and in love, still isn't clear on the ins and outs of vampirism. He knows that high vampires and vampire spawn are different. He knows blood is involved. He knows something about tadpoles. But sifting through the rest of it eludes him. He's asked Astarion a few questions, but it still fits badly with Fenris' Thedosian understanding of the world.

Still, from Astarion's tone, it's clear this is a bad thing. Fenris reaches to trace the green glow in Astarion's palm. Yes, it is fading. "You won't be able to go into the sun?"

It's his best guess, and the thing Astarion prides himself on the most.