Cthulhu is the ultimate bad guy. He makes Darth Vader look like a snotty-nosed 10 year-old.

To explain further...there was once a writer of horror pulp fiction known as H.P. Lovecraft. He wrote a series of lurid tales which some people find scary, while I find them in one sense too verbose and tedious....and in another sense utterly hilarious. I think that Lovecraft was one of the great unintentional humorists of all time. One bizarre story was set in Antarctica, as I recall, and involved some inexpressibly ancient ruins and some gigantic penguins who could shoot electrical bolts out of their heads. These quasi-penguins waddled about the landscape uttering an incomprehensible cry that (according to the narrator...who of course was soon driven irretrievably mad...as happens to all the protagonists in Lovecraft's tales...) where was I? Ah yes, the cry vaguely sounded like "Tek-E-Li-Li".

Now I ask you, what could be more horrifying than that?

But I digress. Lovecraft created a mythos of an incredibly ancient time prior to the present age, in which the "Great Old Ones" ran the Universe. Cthulhu was the Big Daddy, the Grand Enchilada, Numero Uno among the Great Old Ones. They could destroy entire worlds with a mere twitch of a psuedopod, and frequently did.

Lovecraft's great fear was that these Old Ones, with Cthulhu leading the charge, would be reawakened by some bumbling fool and would turn our entire reality on its ear, with results too hideous to even describe, let alone contemplate.

My favourite Lovecraftian passage is:

"Terrible beyond description was the change that had come over my old friend Crawford Tillinghast..."

The unfortunate Crawford Tillinghast (what a name!) later "ran screaming from the room", which is something all Lovecraftian hero/victims do at some point, prior to their final, awful, incomprehensible demise.

Does that do it for you?