The Darkness variant is typically associated with shadows, cold and dark places, hallucinations, and the sort. There's a ton of spook stories in mythology of ghouls and demons, and plenty of evil characters in the realms subject to karma, and the Darkness Valkier basically carries them out. She haunts people, swallows places in darkness, and brings nightmares with the darkness.
She also partners with the Moon Valkier to bring on the night, and once in a while she blots out the light of the moon (an eclipse; Vikings weren't the greatest astronomers so the eclipses could've been deemed as omens or end times.) Even rarer, she may blot out the sun, though that'd be a tough fight with the Sun Valkier to accomplish.

In general, the Darkness Valkier's job is to spook people and make life interesting.

(Again, thanks to @FadingGlory for letting me stick this in the description. X'D)

Ylva in particular is a rather quiet Valkier. Both her personality and speaking voice tend to be pretty monotone, so it can be difficult to tell if she's feeling any sort of emotion, or if she has any at all...

However, she also has a sense of humor, and an odd one at that. Though, if she ever tried to tell a joke, it's pretty likely it'd just go over everyone's heads, due to how monotone she is... She tends to enjoy finding the irony in situations as well.

She looks amazing! It's great to see art from you!

She looks fantastic!! I love how she turned out! So spooky :D

YESSSSSSSSSS THIS IS MAJESTIC

Ylva looks so epic and dark!!! I really love how smooth her body looks, yet also so sharp and deadly~ Fantastic job!!

so beautiful!! Amazing work!

Re: Thank you so much! I needed something big for the finale, and drawing all of the Valkiers in one picture was unreasonable, so I drew the one that was practically a combination of them all xD

Omg this is oooo epic

Thanks everyone! ^v^

Re: Haha thank you!! Airiah would stil be warm because she's a Phoenix dragon, at least xD