Nightlock

Nightlock is a plant with poisonous berries that Katniss and Peeta used to win the 74th Games. The berries are infamous for the lethal poisonous juices that are inside. The Nightlock juice functions similarly to hemlock and potassium cyanide and can kill anyone who eats it instantly. Mr. Everdeen cautioned that anyone who ate nightlock would be dead before the berries even reached his or her stomach, suggesting that nightlock juice may be able to seep directly into the blood vessels in the mouth. The only known sources for these deadly berries in the Hunger Games Series is in the arena at the 74th Games and around District 12 and District 13.

74th Hunger Games
Foxface died after accidentally consuming them during the Games when she stole them from Peeta Mellark, believing they were edible. Peeta himself had mistaken them for edible berries, possibly blueberries, that Rue had shown Katniss. Katniss stopped him from eating any, remembering her father's warning, "Not these, Katniss. Never these. They're nightlock. You'll be dead before they reach your stomach." She and Peeta kept some in a bag in case they could trick Cato into eating them. On the last morning of the Games, when Cato was dead, the rule allowing two victors was revoked. Katniss, in response, pulled out the nightlock berries, giving some to Peeta and keeping a small handful herself. They then held them out to clarify their intentions and on the count of three, threw the berries into their mouths. When Claudius Templesmith took back the change, both of the new victors spit out the berries.

Near the end of the movie, Seneca Crane is locked in a room with a table in the center of the room with a bowl filled with nightlock berries. The shot ends before anything else happens, leaving Crane's demise ambiguous but able to inferred, much like his fate in the book. It is thought to be a cruel twist likely ordered by President Snow for allowing both Katniss Everdeen and Peeta Mellark to win.

Later uses
Nightlock is also the keyword for the Holo and when said three times in a row causes the Holo to self-destruct, killing everyone nearby.

The purple poison pills from District 13, given to the soldiers in Mockingjay, were also named nightlock.They were known to have the same effect as the berries themselves.

Real plants similar to nightlock
Their name is derived from the toxic nightshade and hemlock, both extremely poisonous plants. Nightshade berries are similar in appearance to nightlock. Both hemlock and nightlock are known for their immediate effects and are commonly mistaken for edible plants- nightlock for Rue's berries and hemlock for wild carrots.

Nightlock is said to look like an edible type of berry Rue had found in the arena, which may be blueberries, since they fit the same description.

In the movie they used a black berry with blood red juice. Supposedly the costumers were unhappy, since the berries left red stains on the actors' clothing.