Teen Wolf, Season 5

[Stiles and Lydia are almost to Valack's cell when Stiles glances at a nearby cell door and is startled to see Donovan. Panicked, he looks over again and instead finds a monstrous looking gray-skinned creature in his place. Upon reaching Valack's room, he gives Stiles a curious look]

Dr. Valack:
Tell me what you just saw.

Stiles:
[anxiously] Me?

Dr. Valack:
The creature in the previous cell-- the Sluagh. The myth is that they can take on the appearance of the lost souls that have become bound to it. Happen to have seen any lost souls, Mr. Stilinski?

Stiles:
Everyone down here...?

Dr. Valack:
[smiles] Don't give up on us yet. We're all works in progress.

[Lydia recognizes this statement as something Peter once said and frowns]

Lydia:
Where did you hear that from?

Dr. Valack:
Wise words from a former cellmate.