Erodite

In Borderlands lore, Vault Monsters are powerful, alien entities. Usually these are sealed away in Vaults, and a substance called Eridium is often involved in the opening of these Vaults. For the purposes of the Bunkers and Badasses campaign on Ananke, our Vault Monster (current tentative title "The Source") is reliant on a cousin of sorts to Eridium, known as Erodite.
Erodite is a defining factor in an environment still trying to stabilise from its dramatic creation. Due to the water system that The Source utilises (which is a cycle of the The Empty Ocean and the rivers that feed it), Erodite runoff from contact with The Source is abundant in the world. Rivers near their headwaters are packed with it, thinning as it progresses further down river. Creatures that drink from it are affected by its consumption. Plant life responds to it as they absorb the water. Even the rain cycles contain lesser doses of the mineral, causing the effects to carry further inland from the rivers.
When the Source was cast off from its original world, Erodite was what granted it the ability to drift through space unharmed, and the strength to literally grab chunks of passing earth or rock and form an entire world around itself as a cocoon or shell. Concentrated Erodite is powerful stuff - the amount of Erodite that is causing widespread effects on Ananke is a few drops in the ocean, so to speak, compared to the pure stuff on the Source itself.
In its raw form, it can be found as a fine powder, but that's rare on Ananke. Mostly here it is found in a form mixed into the water, given its distribution method, so its usually spotted by a sheen on the waters surface (reminiscent of oil) with a purple glint in it. If the water is churned up, like if it's been through a waterfall or rapids, it is trickier to spot, forced below the surface into a dark purple cloud of sediment. 
Prolonged consumption or contact with Erodite causes mutation, which can take a dizzying variety of forms. The water cult, The Saints of the Source, are the prime example, as they actively seek to do so - members range from slight physical differences to almost alien nature. Even the civilised folk are affected in minor ways. Most will have some kind of visible change, affectionately referred to as their "Tell", such as visible purple veins up their arms, or unnatural eye colour. Animals rarely look as expected, and the ones more affected by it (such as in the areas at the top of the river) are more hostile and protective of the stuff. Plants can grow and behave in weird ways, and crops occasionally mutate into a totally new strain, or grow to staggering size.
In fact, the effects of Erodite are so prominent in the world that our BBEG, as yet unnamed, is funneling research into how to harness its effects in weaponry. Guns are sort of a big deal in Borderlands games, and he is looking to make his totally unique - in part to help him control this planet, but also for revenge on people offworld. As such, Erodite is pivotal to not only our world, but also to our campaign story and the motivation of key people within it.