The Dark Age
A British gentleman comes to Sunnydale looking for Rupert Giles, but is killed by a dissicated creature he calls Dierdre. After the murder, the creature dissolves into blue goo. Giles later identifies the body as Philip Henry, and denies knowing what the tattoo on Philip's right arm is - though Giles has the same tattoo on the same spot on his left arm. Later, Philip awakens and leaves the morgue, obviously possessed by whatever had taken over Dierdre.
Buffy knows something is wrong when Giles misses a planned meeting, instead choosing to remain home and drink a lot. When Buffy finds Ethan Rayne lurking in the stacks, he informs her that it involves something called the Mouth of Eyghon. Giles tries to keep Buffy out of it, but when the demon moves from Philip to Jenny, Giles realises he needs Buffy's help. He explains that he a five friends summoned Eyghon; the demon then killed one of them. Now the demon's back. It's Willow who comes up with the solution - get the demon to move from Jenny to Angel, who already has a demon inhabiting his body, and one that is much older and stronger.