Nimueh's Chalice was a goblet used by the sorceress Nimueh in an attempted assassination of the warlock Merlin.
Prior to the signing of a peace treaty between Lord Bayard of Mercia and King Uther of Camelot, Nimueh poisoned the chalice with a petal of the mortaeus flower with an enchantment to quicken the progression of the poisoning. Then she posed as a servant girl named Cara in Bayard's entourage in order to trade the poisoned goblet with one of a pair meant as gifts for Uther and Prince Arthur during the feast following the signing of the treaty. Nimueh replaced the goblet meant for Arthur, and secretly told Merlin that Bayard had poisoned the chalice in an attempt to kill the Prince. Merlin stopped Arthur from drinking from the goblet and accused Bayard of poisoning it, prompting Uther to instruct Merlin to drink from the goblet himself in order to prove his assertion. Merlin did so and quickly collapsed and had difficulty breathing from the effects of the poison (The Poisoned Chalice).
Merlin indirectly mentioned this event the next time he confronted Nimueh (Le Morte d'Arthur). Merlin also later mentioned himself being poisoned by the chalice in his rant to Gaius (The Once and Future Queen).