Got this from TV Guide:
A lot of familiar faces will be returning. Not only is Amber Benson coming back as a character other than Tara, so might be Eliza Dushku (Faith), Clare Kramer (Glory), Juliet Landau (Drusilla) and George Hertzberg (Adam). "[They're all returning] for a very particular reason that I will not explain to you," Whedon teases.
Anybody know what's up with this? Wacky Halloween episode, per chance?