Unbound series

Natrual enemies--Hellhound and Valkyrie--Their alliance made them more powerful than anyone could anticipate.

Blurb:

Kara is looking for her missing sister -- she doesn't realize that someone is looking for her. Risk is a hellhound, sent by his mistress to capture her. But she may be his only hope of salvation...

Kol Hildr had spent the last century battling hellhounds and warlocks
who sought to usurp his portal's magic. A mighty shape-shifter with the
ability to change into wolf form, Kol made accessing the powerful
gateway nearly impossible—unless you were a witch of Kelly Shane's
talents. She had a score to settle and would do anything to close the

Guardians Keep is kind of a continuation from the book Unbound. This book is about Kelly a powerful witch and twin sister of Kara from the first book.

This is the second book in the series and boy, Lori Devoti kicks it up a notch!