An inserted adverbial phrase.

Volsung of Nichor: A cunning, treacherous schemer. He first appears as Thorgal's competitor in Three Elders of Aran, but apparently dies in the tests they have to face. Actually, Nidhogg saves him to be her servant; his mission is to gain the Girdle of Immortality from the Guardian of the Keys. For betraying her, she transforms Volsung into a toad and keeps him with her trapped in the neverness.

I'd like to know whether "with her" is an inserted adverbial phrase.

Thank you in advance for your help.