Acathla:
Debut -- Becoming part I (Season 2)
Death -- Becoming part II (Season 2)
Acathla is an old demon that was brought forth to swallow the world into Hell. Basically, there are 2 dimensions: the dimension our world exists in, and the demon dimension -- where the demons live. Acathla's purpose is to swallow our world into Hell, thereby destroying it -- like Armageddon, only not the movie.
Anyway, Acathla is discovered inside of a relic that's just arrived at the Sunnydale Museum. The demon has been turned to stone -- ELO, anyone? -- and is basically dormant. Only someone worthy can awaken him and bring about the earth's demise. Angel fits the bill. He tells Spike the history of Acathla -- before Acathla could open his mouth to create the vortex between dimensions, a virtuous knight pierced his heart with a sword. Acathla turned to stone and was buried where no human or demon would ever look, until they decided to put up low-income housing. Angel's words, not mine!
Angel must recite the proper incantation and wear his own blood on his hands in order to remove the sword from Acathla and wake him up. Just as Acathla is ready to swallow the world, Buffy plunges the sword through Angel, which effectively closes the vortex...but not before sucking Angel into Hell. Acathla's just a big dormant stone again.
Acathla's Look:
Actually, kind of cute for a demon. Acathla reminds me of illustrations of Neptune/Poseidon.Anyhoo, Acathla sports horns, big eyes, and he looks like he's sort of wearing a loincloth thing, too.
Debut -- Anne (Season 3)
Died Same Episode
Andy Hoelich is the first vamp to be slain in Season 3...and by none other than the Slayerettes! He crawls out of his grave only to be met by a smug-looking Willow. The first words he hears as the undead are Willow's, "That's right, big boy....Come and get it." Which incidentally, shouldn't be confused with the old Shoney's Big Boy.
Anyhoo, Andy kicks Oz, does a backflip, and knocks Xander into Willow. It's then that we learn he was on the gymnastics team. So the gang sets up a trap for him, which kind of doesn't work, but they're successful nevertheless. Xander gets knocked flat on his back and Cordy topples onto the vamp, smooshing him between her and Xander. Stake-a-riffic!
Might not be that important, especially since it looks like he was probably just buried in a suit.
The ABCs of Sunnydale