ACATHLA. BtVS Demon/Monster/Villain.
An ancient, powerful demon unearthed near Sunnydale in "Becoming," Part 1. After Angelus and Drusilla steal the turned-to-stone Acathla from the Sunnydale Museum of Natural History, Angelus explains to an unappreciative Spike the significance of their acquisition:
SPIKE: It's a big rock. I can't wait to tell my friends. They don't have a rock this big.
Angelus: Spike, boy, you never did learn your history.
SPIKE: Let's have a lesson, then.
Angelus: Acathla the demon came forth to swallow the world. He was killed by a virtuous knight who pierced the demon's heart before he could draw a breath to perform the act. Acathla turned to stone, as demons sometimes do, and was buried where neither man nor demon would want to look. Unless of course they're putting up low-rent housing. Boys...
(two vamps pry it open)
DRUSILLA: He fills my head. I can't hear anything else.
SPIKE: Let me guess. Someone pulls out the sword...
Angelus: Someone worthy...
SPIKE: Mm. The demon wakes up, and wackiness ensues.
DRUSILLA: He will swallow the world.
Angelus: And every creature living on this planet will go to Hell. My friends... we're about to make history... end.
In the same episode the Scooby Gang seeks to understand the nature of Acathla's threat:
WILLOW: Okay, somebody explain the whole 'he will suck the world into Hell' thing, because that's the part I'm not loving.
GILES: Well, the, uh, the Demon Universe exists in a dimension separate from our own. With one breath, Acathla will create a vortex, a-a kind of, um... whirlpool that will pull everything on Earth into that dimension, where any non-demon life will suffer horrible and... eternal torment.
BUFFY: So that would be the literal kind of 'sucked into Hell'. Neat.
In "Becoming," Part 2, Angelus, with the help of information he and Drusilla gain through torture and disguise (she pretends to be his lost love Jenny Calendar) does succeed in removing the sword, and Buffy must, in order to save the world, kill both her former lover and Acathla (who she mistakenly refers to at one point as "Al Franken") with the sword given her by Kendra.