Many of the more obscure elements are toxic, but some make better poisons than others. Thallium was certainly known to one mass murderer: Saddam Hussein. Iraq's brutal Baath dictator liked to use thallium sulfate as his weapon of choice to kill dissidents and activists.
Thallium is a slow-acting poison, which allows for cruel mind games. Saddam's guards and secret service would sometimes grant prisoners a full release after they'd drunk the poison, knowing they would die a gradual and painful death in the days to come.
Image: U.S. Air Force