Tyrion's mushrooms are a poisonous variety of mushroom, found by Tyrion Lannister in the garden of the manse of Magister Illyrio Mopatis in Pentos. The mushrooms grow from a cracked paving tile. They are pale white, with speckles, and red-ribbed undersides dark as blood.
A Dance with Dragons
While staying at the manse of Illyrio Mopatis in Pentos, a drunk Tyrion Lannister discovers seven poisonous mushrooms growing in the garden. Thinking they might be a message from the gods, he picks them, wraps them in a glove, and stuffs them in his pocket. Later at dinner, Tyrion is skeptical when black mushrooms are prepared by Illyrio's cook, but after a conversation about poison, the magister assures him these mushrooms are not poisonous, and eats several.
At some point, Tyrion stores the mushrooms in the toe of his boot. While heading down the Rhoyne on the Shy Maid, an angry Tyrion is tempted to slip the mushrooms into Griff's supper. After being kidnapped by Ser Jorah Mormont, Tyrion thinks he could eat the mushrooms and thereby avoid being taken alive to his sister, Queen Cersei Lannister. Tyrion is later sold to Yezzan zo Qaggaz on the auction block outside Meereen. Having no faith that Daenerys Targaryen could rescue him, he considers using the mushrooms on Penny and himself.
When Yezzan's hated overseer, Nurse, contracts the pale mare, Tyrion attends him, as the other slaves refuse to go near him. Nurse succumbs to the disease, but Tyrion is implied to have hastened his end by poisoning him. Tyrion serves him soup with slivers of mushroom in the broth, telling him "A Lannister always pays his debts." It is unknown whether Tyrion still has any more mushrooms.
Delicious, and deadly.—thoughts of Tyrion Lannister
The pain is not so much, I am told. Some cramping in the gut, a sudden ache behind the eyes, and it is done. Better a mushroom than a sword through your neck, is it not so? Why die with the taste of blood in your mouth when it could be butter and garlic?