Raid boss

A raid boss is a specialized boss which is designed to be fought by multiple players, typically of significantly higher difficulty than normal bosses and requiring specialized strategy and/or loadouts. Within Borderlands and Borderlands 2, raid bosses have the title "the Invincible" and are exclusively optional encounters, and need not be defeated or even fought to complete the main game.

Borderlands
Originally, Borderlands lacked anything which could be considered a true raid boss. All bosses, even optional encounters, were designed such that single players could defeat them. The first raid boss to appear in the Borderlands series was added by a DLC, The Secret Armory of General Knoxx. The optional mission You. Will. Die. tasked players with defeating the appropriately named Crawmerax the Invincible. Though not specifically designed to be undefeatable by a single player, Crawmerax's high difficulty coupled with hard-to-reach vulnerable spots—especially the one located on his back—made him effectively off-limits to most solo players.

Borderlands 2
On its release, Borderlands 2 was advertised specifically as having a specially designed raid boss meant to be fought by four level 50 characters. The boss, Terramorphous the Invincible, was inspired by Crawmerax, and meant to be more difficult than him. Much of the design of Terramorphous references Crawmerax, including the arena used for the fight as well as the mission name: You. Will. Die. (Seriously.)

A second, somewhat hidden raid boss was also included. Vermivorous the Invincible is the most advanced metamorphosis that a varkid can reach. Though it can theoretically be encountered at any level and with any number of players on True Vault Hunter Mode (it cannot spawn on Normal Mode), the chance of a varkid progressing through the necessary metamorphoses is highest with four level 50 players. Vermivorous is also estimated to have health far in excess of Terramorphous.

Borderlands 2 DLC
The DLCs for Borderlands 2 have also included a number of raid bosses. To date, all released DLCs have included at least one raid boss. Accessing the DLC raid bosses requires a payment of eight Eridium, except for the Ancient Dragons of Destruction who require 20, and Dexiduous who requires between 95 and 99.

Captain Scarlett and Her Pirate's Booty
The first DLC, Captain Scarlett and Her Pirate's Booty, introduced two new raid bosses, available after completing the DLC's main missions. The first, Hyperius the Invincible, is a Hyperion Engineer who has "gone mad with power" and was granted Seraph Guardian status. The second, Master Gee the Invincible, is another Seraph Guardian of unknown origin who fights the Vault Hunters alongside a collection of Sand Worms. A third raid boss (or bosses), the offspring of existing DLC boss Roscoe, was planned but ultimately scrapped before the DLC's release.

Mr. Torgue's Campaign of Carnage
The second DLC, Mr. Torgue's Campaign of Carnage, adds a third raid boss, Pete the Invincible. Pete is an upgraded version of an earlier standard boss, Pyro Pete, who has been enhanced with both fire and corrosive attacks, as well as extremely high health. Like the previous two DLC raid bosses, Pete is a Seraph Guardian and will drop Seraph Crystals.

Sir Hammerlock's Big Game Hunt
The third DLC, Sir Hammerlock's Big Game Hunt, adds two raid bosses: Voracidous the Invincible and Dexiduous the Invincible. The former is a Seraph Guardian stalker controlled by a savage chieftan; and the latter is an enormous drifter. Dexiduous holds the record for the highest Eridium cost of any raid boss, and is the only raid boss alongside Vermivorous battled outside of a private arena.

Tiny Tina's Assault on Dragon Keep
The final DLC, Tiny Tina's Assault on Dragon Keep, adds a new raid boss, the Ancient Dragons of Destruction, that consists of four dragons: Boost the Invincible (slag), Incinerator the Invincible (incendiary), Healianth the Invincible (shock), and Brood the Invincible (corrosive). They can be found in a secret area outside of Lair of Infinite Agony called The Winged Storm accessible by putting 20 eridium in a stone bowl. These dragons will not drop any loot until all are killed. They have a chance to drop the Omen, Stinger and the Blockade.