Deathberries

Deathberries, also known as yew berries, are a type of poisonous fruit that can kill a cat in minutes. Sometimes, kits eat them by mistake, though all cats have to be wary of what they're eating. Deathberries come from a yew bush and to Twolegs they are referred to as yew berries. They can be noticed by their bright red color, and are referred to as Night-seeds in the Tribe of Rushing Water; this might be because they bring "night" (death) to whomever that eats them. They are also poisonous to Twolegs if one eats the seed.

Forest of Secrets
Cloudkit is out looking for herbs with Fireheart and Cinderpaw, and nearly eats them, but Cinderpaw stops him by barreling into his side.

Later, Yellowfang feeds just three of them to a blinded Brokenstar, because she knows he is on his last life. The berries kill him in seconds, showing just how deadly they could be.

The Darkest Hour
When Sorrelkit overhears Darkstripe talking to Blackfoot, he tries to murder her with them to keep her quiet. Yarrow serves as an antidote however, as it causes her to vomit up the deathberries. Darkstripe is exiled from the clan.

Moonrise
Deathberries hidden in a rabbit are the first tactic thought up by the Clan cats to kill Sharptooth, in order to stop his reign of terror over the Tribe of Rushing Water.

Eclipse
At the Moonpool, on the night of the half moon, Littlecloud offers Leafpool deathberries, strictly for medicinal purposes, of course. He says that ShadowClan uses them to end a would-be prolonged or suffered death of a sick or grieviously wounded cat. Leafpool politely declines the offer, but Jaypaw is intrigued by the idea of holding a cat's life in his own paws.

Sunrise
Leafpool stuffs a mouse full of deathberries and put it near the rocks, so the snake that had killed Honeyfern would eat it and die. This plan later fails in withdrawing the snake from its hiding place.

Hollyleaf, after revealing to all four Clans that Leafpool and Crowfeather are their parents at the Gathering, tries to feed deathberries to Leafpool. Leafpool reasons with her, saying that she had lost her kits, her one love, and her calling as a medicine cat. Leafpool asks which would be easier, to die or to go on living. Hollyleaf lets her go, knowing she will be more anguished living.