Buffy The Vampire Slayer (1997) - 4x22 - Restless

Buffy, Xander, Giles and Willow sit down to watch videos after defeating Adam. They enter a bizarre dream world where they are pursued by a primitive force that they released via their enjoining spell. The force turns out to be the spirit of the first Slayer who speaks through Tara in their dream world and is confronting each of them with their deepest secrets and fears before killing them. Buffy eventually dispatches the primitive girl by ignoring her and they all wake up.