The Hero obtains Hist Sap during the Blackwood Company mission "Infiltration." It causes them to teleport to the quest destination of Water's Edge, where they must kill all of the goblins infesting the location.
It is only later that they find all but one of the residents of Water's Edge is dead, and realize that the Hist Sap made them think the residents were goblins. This explains how, when entering a house with a 'goblin' in, they do not attack.
- Even if the Hero is an Argonian, the hist sap still affects them for some reason, which is referenced by Modryn Oreyn: "Based on your heritage, I'm surprised it affected you this way, it must be different sap."