Crossover with Buffy the Vampire Slayer.
Spike was prowling Sunnydale for prey, when he finds a mysterious door leading to a more colorful world, the world of Equestria. After going through he realizes he can't get back through, and learns that the portal between worlds closes and opens periodically. he now has to wait until the portal re opens to go back. takes place in season 4 of Buffy before Spike receives his behavior modification chip, and right before the first episode of My Little Pony