Othalanga, the suicide tree.

WaPo:

The thin-branched, flowering tree bears a deadly harvest: a softball-sized fruit with seeds so toxic they can stop a heart. In the 19th century in Madagascar, where the tree is also found, thousands of people per year died after consuming the seed in �trials by ordeal� believed to determine whether they were guilty of witchcraft or other crimes. And a 2004 study found that it�s responsible for roughly a death per week in Kerala, most of them suicides. Researchers believe that more people have taken their own life using othalanga than any other plant in the world.