You might be thinking of the Norse myth in which Loki tricks the blind god to shoot an arrow of mistletoe at Baldur, as it was the only thing Baldur's mother overlooked when she went around asking everything not to harm her son. Loki found this out, and at a throwing contest in which Baldur stood in the center, letting the gods throw knives and such at him since he could take it without so much as a scratch, Loki handed his arrow of mistletoe to the blind god and encouraged him to give it a try. The god threw it, and it made its mark, killing Baldur instantly. This is what got Loki bound to the wall of a cave with snake venom dripping on him. If there is a Greek myth in which mistletoe is used in the same way, I am unfamiliar with it.
