Basilisk's blood is found within the Basilisks of Yi Ti or the Basilisk's Isles off the Northern Coast of Sothoryos.

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According to ASOIAF, the poison consists of a paste-like texture, which is usually spiced with the Basilisk blood to produce the venom.

This venom is said to contain a savory smell, but when ingested it can cause a violent madness in any creature with warm blood, humans or animals.

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Mentions:

The Waif tells Arya Stark that a mouse will attack a lion after a taste of basilisk blood.

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"Basilisk blood. It will give cooked meat a savory smell. But, if eaten, it produces violent madness... in beasts as well as men. A mouse will attack a lion after a taste of basilisk blood. But to a man who knows his heart, is not ion a form of madness?"

- Oberyn Martell

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Source: Wiki