Agrabahn Viper poison is a poisonous substance created from the venom of Agrabahn Vipers. If a person drinks the poison, they die almost instantly and then their body disappears in a cloud of green smoke.
After the Second Curse
Arthur convinces Grif to kill himself for Camelot's sake. ("Siege Perilous")
As he doesn't trust the Storybrooke folk for lying about Emma being the Dark One, Arthur plans on making the town the new Camelot for his people, all the while gaining David's trust. As part of the plan, he tells his squire, Grif, to steal from a relic chest. Arthur lies to David about someone taking a bean which could take his people home, so David examines the chest which was broken in to. He asks Grif for any theories on who did it, and without the sheriff noticing, Arthur gives Grif a knowing look, to which the squire talks about Emma being the culprit. David thinks it's possible since she just stole an axe, but Arthur points out that she wouldn't need magic and the chest has scratch marks on it. At the pawnshop, they ask Belle if anyone pawned in a magic bean, but she answers negatively. David improvises and takes an ordinary cup and announces to the people of Camelot that they must drink from it to find the thief. Grif flees on horseback, causing Arthur and David to follow him in the latter's truck. Arthur takes over the wheel while David knocks the squire off his horse jousting style. Grif "confesses" to stealing the chest, claiming he was sick of the way Arthur treated him, but claims there was never a bean. David privately asks Arthur if he thinks Grif is lying, but he says he believes him. David promises to find another way home and then prepares to lock Grif up when they discover a toadstool that came from Camelot during the curse. Later, Arthur visits Grif in his cell and convinces him to ingest a fatal poison for the good of Camelot, so the townspeople will never learn the truth of what happened. ("Siege Perilous")