Eridium is a new element found in Borderlands 2. It was released onto Pandora after the Vault Hunters opened The Vault. 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 other elemental 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 catch-all power source, as the Eridian weapons from Borderlands use Eridium chunks as a reusable and rechargeable ammunition source.
Eridium and Sirens
Due to the mysterious relationship of Sirens to Eridians, Sirens can absorb Eridium as a means to enhance their powers. However, this has serious pros and cons. 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.
Bars of refined Eridium are used as a means of underground currency, and can be used to buy character upgrades at the Black Market. These include Bank, Backpack, and Ammunition SDUs. The black market can only be accessed after commencement of the Plan B mission. A secondary use is for fighting raidbosses; it costs four Eridium to fight Terramorphous the Invincible, eight to fight Hyperius the Invincible, Master Gee the Invincible, and Pete the Invincible and 90 - 99 to fight Dexiduous the Invincible.
Eridum itself is also used as a very powerful fuel source, and possess unknown properties as well. However the exact characteristics of the matter is unknown. While it seems to be almost like energy made solid, refined eridium ore is formed into ingots, a process normally done for metallic materials. Eridium based weaponry often possess a slag effect as well.