At the end of the murder mystery questline, you will be asked to open a chest. In that chest, you will find the Mysterious Amulet and upon wearing it you can start seeing invisible Eridium piles.
There is a little room in Carnivora in which you can find 3 Eridium piles each time you reload the game. Depending on your luck, some of them can be invisible. You can see the location in the video below. Note that, I advise you to play on higher Mayhem levels to obtain more Eridium.