Mercury is a metallic, chemical element that will result in death after prolonged exposure. 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 enter the Avatar State to fight off the poison, which would enable the Red Lotus to kill her in the state. Although the poison should have been lethal, Korra was able to fight it off.
In an attempt to bring forth worldwide anarchy, the Red Lotus intended to permanently end the Avatar Cycle. Having captured Avatar Korra, they administered the poison, which immediately produced a physical and mental reaction. As a side-effect, the poison caused Korra to hallucinate, seeing the Red Lotus members' faces turn into those of Amon, Unalaq, and Vaatu. Although she attempted to resist the poison's effects, 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 captors. 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 she told her that the venom was metal-based; Suyin promptly removed most of it from Korra's system, saving the Avatar's life.
Nonetheless, the poison had caused significant internal damage, rendering Korra unable to walk for nearly six months and incapable of contacting Raava or entering the Avatar State. It even obscured Raava's energy for other spirits. In addition, the residual traces of poison in her body prevented her from making a full recovery. With the help of Toph Beifong, Korra was made aware of the metal remnants in her body. Although she subconsciously resisted the removal of the metallic poison at first, she eventually managed to let go of her fears and bent the substance out of her body, which Toph subsequently contained in an earthen shell.
The only known way to administer the metallic poison is by metalbending it into the victim's pores. Once administered, it has a 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 the Avatar, the Avatar State will naturally react to the lethal threat and be activated in an attempt to fight it off.