There is a boss in Borderlands 2 so tough you probably won't be able to kill it. But there's a quest to kill it anyway. It's called 'You.Will. Die. (Seriously.)'

That's Terramorphous the Invincible. Good luck.

The boss is called Terramorphous the Invincible, and he earns his name. This gargantuan thresher serpent thing emerges from the rock with a roar that echoes throughout his cliff-side home, Terramorphous Peak in the Thousand Cuts area. Its knock-back attack sends you flying high into the air, probably to your death. It hits hard enough to kill you outright. Its tentacles, many of which protrude from the ground, nag at your already frayed concentration. And the number of hit points it enjoys... well, let's just say, there are enough of them to make the player feel very, very insignificant.

"If you don't curse the design team at least five times on the way to beating this guy, we haven't done our jobs," says producer Randy Varnell with a smile.

Terramorphous was conceived after Borderlands developer Gearbox Software saw how players enjoyed taking on Crawmerax the Invincible (no relation), the crab worm raid boss from the superb The Secret Armory of General Knoxx downloadable add-on. The mission to kill that beast was called 'You.Will.Die.' You see what they're doing here.

Crawmerax was pretty much the hardest challenge the first Borderlands had to offer, and required a group of four maximum-level players all with high-level gear to take it down. Terramorphous is like that, except he's even more of a bastard.

A raid boss comes on the disc this time because Gearbox wants to include enough end-game content to keep players who've hit the level 50 cap playing for years after launch. The developer has already revealed one of the new mechanics designed to do this, the unique Badass system and its infinite levels of profile-wide progression. We're at Gearbox's Dallas headquarters to see the rest.

"If you don't curse the design team at least five times on the way to beating this guy, we haven't done our jobs" - producer Randy Varnell.

"Once you hit that end game, what are you doing? That's what Crawmerax was," Varnell says.

"You can fight Terramorphous multiple times and he's going to drop different loot every time. Some of the best gear in the game, great legendary items, will drop from this guy. He has specific items that only drop from him in the game. There are some legendaries you'll only find from him.

"A lot of the end-game pull is that four-player co-op challenge at a high level against a really tough-to-fight boss. He is designed to be impossible. I'm not going to say he's not soloable, because I know how well gamers break games, but he really is tuned for that end-game, four-player co-op fight, testing your build and your gear and your equipment and your ability to work together as a team. His rewards are great. So, if you are that end-game player, this is your challenge right here."

To the left you can see Maya use her Phaselock ability. Put points into the healing tree and you can use it to revive downed party members - incredibly useful during boss fights.

One end-game raid boss, though, won't be enough to keep the hardcore happy. Varnell mentions "game changers". Each character class has one: a unique ability deep down at the end of a skill tree, only available at level 50 and designed to, as the name suggests, change the way you play.

Take Axton, Borderlands 2's soldier, for example. His base skill sees him deploy the Longbow Turret. But progress all the way down his skill tree and you'll unlock the Nuclear Detonation ability, which triggers every time you deploy the turret. It looks as over-the-top as it sounds, complete with a modest mushroom cloud effect. Combine this with the Longbow skill, which teleports your turret to wherever your crosshairs are aiming, and you have a pinpoint-precision nuke at your disposal. Combine all that with the skill that lets you pick up the turret after it's been thrown (which reduces the cooldown on your nuke), and you can get into a faster, devastating loop of throw, nuke, pick-up, throw, nuke, pick-up. "It changes up the dynamic of the soldier quite a bit from just a turret shoot type of class," Varnell says.

Take Zer0, the Assassin class, as another example. Zer0's hook is that he can disappear - as you'd expect from a rogue type - allowing him to stealth around the battlefield and attack with enormous bonuses to damage. Make your way to the bottom of the Bloodshed tree, though, and you unlock skills that let you stay in this Deception mode potentially indefinitely. From Deception, make a dash kill within five seconds and you reset the countdown. So, you can, if you're good enough, chain kill after kill after kill without decloaking. It's tricky, though. In the Playground, Gearbox's unhindered developer environment used for testing Borderlands' new toys, marketing chief Adam Fletcher does well to chain four kills.

One of Salvador's game-changing skills is quintessentially Borderlands, and quintessentially Gearbox. At the bottom of the Brawn (tanking) tree is the 'Come at me, Bro' skill. Salvador gives the enemy the finger, with both hands, aggroing the lot (you'd be annoyed, too). That sounds like bad news for Salvador, but Come at me, Bro instantly brings him up to full health and reduces damage taken by 90 per cent for a few seconds. Occasionally, Salvador laughs maniacally. He likes the attention.

And finally there's Maya, the Siren class. Work your way to the bottom of her Harmony tree and you'll unlock Scorn. This sees Maya throw out an orb of energy that slowly flies past enemies, covering them in slag, Borderlands' inappropriately-named substance that increases damage taken. The trick is to avoid hitting enemies with the orb - harder than it sounds - and maximising the number of bad guys it flies past - also tricky. Nail it, though, and you can take down even the toughest enemies with just a few shots.

At level 21 Salvador can unlock the Fistful of Hurt ability, which changes the standard melee attack into a davastating uppercut that sends enemies flying. Great for getting back agro when a party member is in trouble.

"True Vault Hunter Mode is more than just difficulty. You now need to use all the tools and all of the tricks you learnt in normal mode, and that's how you survive" - lead designer Jon Hemingway.

Powerful skills are nothing without powerful enemies to use them against, of course. While Borderlands 2's normal difficulty will present a decent challenge to players, it's more about teaching the player how the character classes work, how the enemies work and how the guns work in a fun 40-odd hour blast 'em up. It's not until you beat Borderlands 2 on normal that you'll pit your skills against a real test.

Borderlands 2's second play-through is called True Vault Hunter Mode. Like the second play-through on Borderlands, TVHM is harder with tougher enemies. But where the first game simply tasked the player to increase their hit points and do more damage with better guns against enemies with more hit points and better guns, a second play through of Borderlands 2 requires the player to think. The AI works differently using unique behaviours. More enemies use shields. More wear armour. More are Badass class. There are enemies that only appear in TVHM. To defeat them you'll have to put those top-tier abilities to good use. And in co-op, which automatically scales up the challenge depending on how many are in your party, understanding how different classes work together is essential.

Varnell has a couple of examples. A skilled Maya player can use the Scorn ability to fire off an orb so it just misses all the on-screen enemies, slagging them all. Then it's up to the other players to do the damage. Salvador can use Come at me, Bro to taunt all the enemies in the battlefield. Then, just at the right time, Axton deploys his turret with the Nuclear Detonation ability in tow. It's the only way to be sure.

"With all the skill trees, we always have the fun button, as we call it internally, where you do something cool," Varnell explains. "But for the core guys, you're seeing all these additional tactics are coming up. We want the guys who are really deep into game to feel reward for having these new, more advanced skills available to them."

Some Borderlands fans may be sceptical of the challenge offered by True Vault Hunter Mode. Those who rinsed the end-game will know that on the second play-through, the enemy scaling was a little off. In short, eventually, players overpowered the environment.

"That is not a problem this time around!" lead designer Jon Hemingway tells me when I follow this up with him. "True Vault Hunter Mode is more than just difficulty. You now need to use all the tools and all of the tricks you learnt in normal mode, and that's how you survive.

"We all felt it needed to be more than just turning up the dials. More than just more damage and more health and ta-daa we're done. For example, more enemies have shields. That seems like a really small thing, but it now makes shock more viable. So if you're playing in co-op, get someone with a shock gun and their job is to bring down the shields. Enemies will have more armour on them. So don't shoot them in the armour bit. You're going to have to aim a little bit better and hit the sweet spots.

"We've spent quite a bit of time thinking about it. We've given you a whole chest of tools. Now, if you want to survive, use them all. First game, you don't need them all. Have fun, but learn them all. And now it's time to separate the men from the boys. That's what True Vault Hunter mode is all about. It's much more than difficulty. It's tactics."

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Gearbox ends its show with a futile attempt on Terramorphous' life. First, Fletcher tries to solo him as Axton. A powerful attack sends him flying high into the air and off the map. He's dead. He tries again, this time with a friend. They last a minute or so, the huge tentacles and devastating attacks making short work of their pointless effort. I glance at the beast's life bar. It hasn't changed.

But Terramorphous can be killed if you're working in a group that knows what it's doing. "You'll need a Siren in your party that has the ability to heal," Varnell says, "and a Gunzerker that has the ability to taunt and take damage from big attacks. Watch for the tells, knock-backs and big attacks. Like all great raid fights, you learn the rhythm of the fight and where he is and develop a strategy against that."

"We're all huge Diablo fans. You can't make a game like Borderlands without having played Diablo" - lead designer Jon Hemingway.

Gearbox has made no secret of the influence Blizzard's loot-hungry Diablo series has had on Borderlands. The first game was, at its base, Diablo with guns. Four character classes. Four-player co-op. More encounters than you can count. And more guns than any game ever released. "We're all huge Diablo fans," Hemingway tells me. "You can't make a game like Borderlands without having played Diablo.

"Some of the obvious similarities and things we borrowed from were the loot. Kill guys, get better loot. One of the other ones is defining who you are. In Diablo 2, lots of terms started to emerge as people found similar builds. There's the Amazon class. If you used a javelin with her though, you were a Javazon. People came up with a whole language for identifying players. We started to see things like that.

"Around the office right now, one of the names for the common builds for Zer0 the Assassin is shotgun ninja. Grab yourself a good Jacob's shotgun, stealth in behind them and backstab with a shotgun. It's spectacular fun."

Most of us loved Borderlands for the same reason we loved Diablo: the unending loot, the unending desire to optimise your character, and the fun you had doing it. People are still playing Borderlands some three years after launch. Gearbox hopes Borderlands 2's end game will keep you hooked for closer to a decade. Just expect to die trying.