In the "Hunger Games" trilogy, Katniss Everdeen plans to use Nightlock as a last resort if she finds herself in a situation where she cannot escape or believes that her capture by the Capitol is imminent. Nightlock is a poisonous berry that grows in the arena and is deadly if ingested.
Katniss first encounters Nightlock during the 74th Hunger Games when she and Peeta are being pursued by the Career Tributes. They come across a patch of Nightlock berries and collect them as a potential means of self-defense or escape.
Throughout the series, Katniss sees Nightlock as a symbol of rebellion against the Capitol's control. She believes that if she were to be captured by the Capitol, they would use her as a propaganda tool to quell the uprising. In that scenario, she plans to use the Nightlock berries to defy the Capitol and deny them the satisfaction of manipulating her.
Katniss discusses her plan to use Nightlock with her allies, including Gale and Peeta, making it clear that she would rather die than be used as a pawn. However, she hopes to avoid such a scenario by staying alive, fighting against the Capitol, and ultimately bringing about its downfall.
While Katniss contemplates using Nightlock as a form of defiance, it is worth noting that in the final moments of the series, she finds an alternative solution to the challenges she faces, leading to the resolution of the story.