Buffy the Vampire Slayer: Season 8

On a quiet night in the Slayer's fortress, a sudden attack catches Buffy and her forces completely off-guard. Vampires with the ability to become animals and mist neutralize key people in the castle, and steal the Scythe that allowed Buffy to awaken all of the potential Slayers.

After a moments thought, they realize where they've seen a vampire with these precise abilities before. A very old enemy has set his sites on Buffy.