Eridium

Eridium, not to be confused with Iridium, is a new element found in Borderlands 2. It was released onto Pandora after the Vault Hunters opened The Vault in the events of Borderlands. Along with the discovery of Eridium, Slag, a by-product of refining Eridium, has been used for weapon manufacture due to its unique properties.

Eridium can be used by NPC Sirens to augment their own powers.

The maximum number of Eridium bars a player can have at any one time is 500.

Eridians and Eridium

Eridium is appropriately named after the ancient precursor race that utilized it, the Eridians. The ore has some semblance of usage as a universal power source, as the Eridian weapons from Borderlands use Eridium chunks as a reusable and rechargeable ammunition source. Neither the Eridians nor their weapons are present in Borderlands 2.

Eridium and Sirens

Lilith absorbing Eridium

Due to the mysterious relationship of Sirens to Eridians, Sirens can absorb Eridium as a means to enhance their powers. However, this has serious side effects. While Eridium increases a Siren's powers and makes them more powerful, even increasing their natural healing to the point of superhuman regeneration, the 'high' of the power boost has the potential to be addictive. Sirens that are given, willingly or forcefully, a constant flow of Eridium to absorb become unable to live without it: the Eridium essentially becomes their life support. Should the constant flow of Eridium stop, the Siren will die.

Despite this, Angel mentions in an ECHO recording that Maya (the only playable Siren in Borderlands 2) has no connection to Eridium.

Uses

In game, Eridium can have many different uses:

• Buying character upgrades at the Black Market. These include Bank, Backpack, and Ammunition SDUs. See Black Market page for further details. During Plan B in Normal mode, a Vault Hunter must make a purchase to continue the mission.
• Accessing Terramorphous Peak for fighting Terramorphous the Invincible, costs 8 Eridium for each access.
• Fighting Hyperius the Invincible, Master Gee the Invincible, Pete the Invincible and Voracidous the Invincible, costs 8 for each fight.
• Spawning Dexiduous the Invincible in Hunter's Grotto, costs 21-27 Eridium to activate each of the four shrines.
• Accessing The Winged Storm for fighting Ancient Dragons of Destruction, costs 20 Eridium for each access.
• Playing Tiny Tina-themed slot machines in Flamerock Refuge, costs 2 Eridium for each pull.
• In Tiny Tina's Assault on Dragon Keep, activating shrines for a short durating of combat buff, costs 2 for each activation.
• Using two dice instead of one when playing 'd20' chests, costs 5 Eridium.
• At the end of A Game of Games, a Vault Hunter must feed Butt Stallion 5 Eridium to continue the mission.
• After finishing A Game of Games, feeding Butt Stallion in Flamerock Refuge will cause her to dispense an item in return. It costs 5 Eridium for each interaction.

Eridium itself is also used as a powerful fuel source; however its method of production and full properties are unknown. While it seems to be almost like energy made solid, refined Eridium ore is formed into ingots. Eridium can also be used in weaponry, found in the form of E-tech gear possessing unique projectiles and improved elemental abilities.

Drop Guide

Eridium is an uncommon-rare drop from regular enemies and rarely found in containers like lockers and chests. Some bosses drop Eridium more frequently. It can also be obtained by playing slot machines around Pandora, or as a mission reward.

Early in the story missions, if the game is saved and quit after entering Bloodshot Stronghold, an exit back to three horns divide will spawn Badmaw again for a high chance at getting Eridium. Later in the story missions BNK3R, the Warrior, and the Ancient Dragons of Destruction can all drop high quantities of Eridium.