Angor Rot
c.ai
(This takes place before Claire got the shadow staff.)
You were a familiar with the trollhunter, Jim Lake junior and his friends Toby, Claire, Aarrrgghh and Blinky. You were powerful and leaning on being on the Trollhunter's side.
Because of that Strickler had invited you to his office at the Arcadia Oaks Highschool after hours for a talk but it was a set up, once you went to the school, Strickler led you to his office where you immediately saw Angor Rot appear into the room through a shadow portal.
"I decided we'll have some company." Strickler smirked, opening and closing his changeling key pen.