Birch sap

The Birch sap is a concoction the Treeminders of Oasis give to outsiders as part of a purification ritual before they are allowed to meet The Great One (also known as Harold). The Vault 101 Dweller will be rendered unconscious after drinking the sap and wake up in the Grove some time later. According to Harold the Birch sap have varying effects, amongst which hysteria seem to be common. This is what seems to have happened to the Wasteland Merchant that you can meet as a random encounter, who will speak of the Treeminders when you open a dialogue box. Once the conversation is finished, he will die immediately, and you can loot his corpse.