Borderlands 2's hardest enemy (so far) is Terramorphous the Invincible.

Four team members from Gearbox Software went up against Borderlands 2's toughest boss monster in a demo at PAX today. With four max level characters and an armory of high-level guns — and after cashing in a few "Bad-Ass Rank" tokens to further boost their skills — the players sounded the horn that summoned Terramorphous the Invincible.

Working in tandem, they spent a good five minutes firing bullets, launching rockets, and firing sentry guns at Terramorphous. The developers did about 5% damage to him before dying a variety of gruesome deaths. It wasn't even close.

Terramorphous the Invincible attacks players with a seemingly endless supply of tentacles, attacking them from below. He also packs a deadly ground pound attack that does huge damage. It also knocks players back and into the air, often into the pit from whence Terramorphous came. It's a fatal fall.

The loot that will drop from the corpse of Terramorphous is said to be some of the best in the game, however, so players may find the challenge worth their while.

Gearbox president Randy Pitchford said the developer put Terramorphous in Borderlands 2 to offer players who had already reached the game's level cap of 50 something to reach for, something that would outdo the extremely difficult Crawmerax from the expansion to the original Borderlands, "The Armory of General Knoxx." (The mission to kill Terramorphous is titled "You. Will. Die. (Seriously.)," a callback to the original Crawmerax mission.)

Pitchford said Gearbox wanted to include an end-game raid boss in the main game instead of downloadable content to ensure that every Borderlands 2 player got a crack at Terramorphous.

Despite the challenge, despite Gearbox developers' failure to topple Terramorphous, Pitchford says he's not actually invincible. He can be beaten. And Pitchford tells Polygon that one Gearbox employee has beaten Terramorphous solo, by weakening the tentacle-like stalks to their minimal hit points, then taking advantage of Borderlands 2's "Second Wind" feature that restores a player's health when they're in danger of dying.

Vault Hunters will have a chance to take on Terramorphous the Invincible when Borderlands 2 launches on September 18th on Xbox 360, PC, and PlayStation 3.