Plague blossom (also called num’chadran and failure flowers) is an engineered plant designed by The Raven King’s real name. Bran’s grandfather, he’s an incredibly dangerous creature, and one of the few old enough to know Notte from Notte’s infancy. He’s got a big role... More the See Kanon, Kalarastri, The originator of the Unseelie Fey. Mab spent a good long time working with Notte in his emotional infancy. Brilliant and capable, her sacrifices and leadership are the only reason... More, and Faelan to try to combat the fallout from the First War, which poisoned the soil and water and air badly enough that almost all plants and crops in affected areas died.
Unfortunately, it only worked part of the way; it successfully began to draw the poison from the soil, but then something went wrong. Instead of breaking down the toxins, the plague blossoms managed to refine and condense them, creating a far more dangerous substance which dripped from their leaves.