Katniss and Peeta nearly take nightlock at the end of "The Hunger Games." Nightlock is a fictional poisonous berry known in Katniss’s home district, District 12.
After the Gamemakers revoke the rule allowing two winners from the same district, Katniss and Peeta refuse to kill each other. Katniss produces the berries and proposes that they eat them together, leaving the Hunger Games without a victor. The threat forces the Capitol to reverse its decision just before they swallow the poison.
Nightlock also causes Foxface’s death earlier in the story. Peeta gathers the berries, not realizing they are deadly, and Foxface later eats them after stealing them from his food supply. This is why Katniss immediately recognizes the danger.
A common mix-up is calling nightlock an ordinary poison or a manufactured drug. In the story, it is specifically a lethal berry. The scene is also more than a suicide threat: it is Katniss’s first major public act of defiance against the Capitol.