Café de Nuit

In a campaign setting where only the Nine Hells exist, what horrible creature would guarde the gateway?

Something that will both keep out unwanted corporeal travellers as well as allow the passage of souls into the plane.

I liked the idea of Cerberus, who guards the entrance to Hades. However I was thinking about something with a more fiendish air. For a time I was considering something Lovecraftian. Something that even greater devils would feel uneasy around, but I dunno..

Suggestions welcome.

I liked the idea of an Umbral Blot (see Epic Level Handbook). A truly effective guardian would be more like a force of nature, and an Umbral Blot seems like a barely-contained mobile sentient black hole.

And nobody wants to mess with a mobile sentient black hole.

Mythologically speaking, guardians of the underworld were often represented by dogs: Cerberus, Anubis, St. Christopher, the dogs buried with Celtic sacrifices. These were the psychopomps: guardians of the common gateway rather than judges or grim reapers. If it's a gateway guardian you're looking for, I'd recommend that theme.

You could take the dog angle, and twist it severely.
The entrance to Hell is inside of a shadowy canyon, and as the players approach, a hound emerges. It's bloated body is nearly the size of a hippo, and all three slobbering heads face out, with the one on the right biggest and the other two getting smaller on the way left.
The massive thing darts back and forth from the canyon's gloom, although sharp eyed characters might note that it seems to move to far, too fast for its legs.
Then, as the characters approach closer, they see the hindquarters of the beast - a hairy shaft as thick as the body, running back and up into the shadows.
And then, It[i/] emerges. The horrible gaurdian of hell, of which the hound is but its forepaw!
And as it bears down upon your hapless players, they will know that the indescribable terror does not but keep the living from Hell, it also keeps the devils from escaping.