Mystery of the Nightmare Venom is the second adventure in the Nightmare Venom campaign for the prehistoric D&D Planegea campaign setting. It is designed for four to five 2nd-level characters using the D&D 2014 ruleset. In the first campaign adventure, Venom Attack!, the PCs encountered an envenomed spider-raptor sent by the Spiderfolk Queen to attack Edgegather, while a minion of a green hag sought to collect a sample of the venom. In Mystery of the Nightmare Venom, the PCs begin to investigate this strange magical poison at the behest of the Venom Guard: A vengeful green hag seeks to corrupt the inhabitants of Edgegather. Can the PCs stop her before her poison brings the settlement to its knees? The goblins of the Unfalls may have useful information but the PCs must first retrieve a lost relic from the spiderfolk of the Venom Abyss before they can ask their questions. The PCs investigate a vast and ancient temple beneath Edgegather where the mysterious Veiled Goddess can point them towards the Oracle of Seerfall. Mystery of the Nightmare Venom contains five new monsters (the envenomed chattercrest, the envenomed Venom Guard, the cauldronborn, the Glassback spiderfolk, and the faithful remnant) and two new magic items, the bloodtallow candle and the obsidian centipede.