how do i describe dark/ evil feeling?

Does evil feel tempting in your story? Then perhaps your character smells her favorite food. Perhaps the demon is incredibly alluring.

Does evil feel like danger in your story? Then perhaps your character has a cold sweat as she hears this, she smells the scent of smoke.

Does evil feel like corruption in your story, and this corruption is blatant? Then perhaps your character smells decay.

Use sensory details to give your readers the feeling of evil.

How would YOU feel if a demon offered all that?  I presume you are human, and you say your character is human. 

You know your plot and we don't. You know your character and we don't.  So you know what she would do. I also presume that anyone who works for a vampire is not squeamish about encountering demons. She has to deal with removing spilled blood and bloodstains from carpets and furniture. (BTW, does she steal some blood from the blood bank for her other boss?)  If she thinks the demon's offer is a bad deal, she'll tell the demon to get lost in the name of Jesus.

Why are her parents idiots for letting a demon in to see her daughter?  Do they want her to own a mansion and be the devil's soul-mate?  It would get her out of the house, and they were probably offered perks in return.

Since very few people will bother to decipher your meaning from your poor spelling, you will be writing your story for yourself.

Ideally you would learn English before attempting to write the scene in English. Perhaps you could make a deal with some infernal being to acquire English at a faster pace.