Mercury



The poison used by the Red Lotus is a metallic-based substance that, when exposed to it too long, will result in death. It was used on Avatar Korra in 171 AG in an attempt to end the Avatar Cycle for good; as a natural reaction, she would go into the Avatar State to fight off the poison, which would enable the Red Lotus to kill her in the state.

History
In an attempt to bring forth worldwide anarchy, the Red Lotus intended to stop the Avatar Cycle for once and for all. Having captured Avatar Korra, they administered the poison. Korra tried to resist going into the Avatar State, the poison making her hallucinate, seeing old enemies in the form of Amon, Unalaq, and Vaatu, though she eventually caved in and she was forced into the Avatar State. Before she could be killed, however, the surge of power she received from Raava's spirit enabled her to break free from her chains and fight off her captives. She subsequently locked into battle with Zaheer, though Korra found herself struggling to fight both him and the poison coursing through her veins, which gave Zaheer the upper hand. With the help of the new airbenders, she was able to overcome him, but she succumbed to the poison nonetheless, her body having been exposed to the lethal substance too long. Jinora spurred Suyin to action, however, when the young airbender revealed that the venom was metal-based and Lin's younger sister promptly removed it from Korra's system, saving the Avatar's life. The strain the poison had put on Korra's body was still grave, though, as even two weeks later, the Avatar had not recovered from its administration.

Effects


The metallic poison is administered to the body by metalbending it into the victim's pores. Once administered, it has an hallucinogenic effect. After that stage, the victim will be slowly but surely hindered in their movements until the strain on their body eventually becomes too painful to bear and they will slip into unconsciousness, eventually resulting in death.

When used on an Avatar, the Avatar State will naturally react to the lethal thread and be activated in an attempt to fight it off.