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 quest You. Will. Die. tasked the player 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 a more difficult than him. Much of the design of Terramorphous references Crawmerax, including the arena used for the fight as well as the quest 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, 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 DLC for Borderlands 2 has also included a number of raid bosses. To date, all released DLCs have included at least one raid boss. Unlike Vermivorous or Terramorphous, the DLC raid bosses cannot be fought without limit; instead, accessing each requires a payment of 8 Eridium, and once defeated the raid boss cannot be fought for another 24 hours.

To date, all DLC raid bosses have been identified as Seraph Guardians, primarily relevant in that they will all drop Seraph Crystals when defeated. They can also be encountered as level 32 bosses in normal mode in addition to level 52 bosses in True Vault Hunter Mode, the primary difference being that the level 32 variants will not drop Seraph Crystals.

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 questline. The first, Hyperius the Invincible, is a Hyperion Engineer who has "gone mad with power". The second, Master Gee the Invincible, is another Seraph Guardian of unknown origin who fights the Vault Hunter alongside a collection of Sand Worms.

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 a rather large amount of health. Like the previous two DLC raid bosses, Pete is a Seraph Guardian and will drop Seraph Crystals.