Hemlock

Hemlock is the popular name of a plant known scientifically as Conium, consituting two species of highly poisonous flowering plants in the family Apiaceae, native to Europe and the Mediterranean Region.

The poison derived from such plants was used as a method of execution in Classical Greece. It was considered that letting the condemned person himself drink the hemlock was a more dignified way of dying then having to submit to an exectioner. 

In Western culture hemlock is especially associated with Socrates, who was condemned for impiety and sedition, and allowed an honorable death by drinking this poison as an alternative to exile from Athens.  

Hemlock in "The Daimon"

After seizing power in Athens, Alkibiades came to the regretful conclusion that he had to get rid of Sokrates, who had been his teacher and friend, after the philosopher openly defied Alkibiades' rule. Rather then sending assassins to kill Sokrates (as he had done with Kritias), Alkibiades offered  Sokrates the more dignified option of drinking Hemlock. However, since Alkibiades was a tyrant who had seized power by force, overturning the laws of Athens, Sokrates did not feel bound to drink the poison. On the contrary, feeling as his civic duty to oppose the usurper to the best of his ability, he pushed the hemlock out of Alkibiades' hand and nearly strangled Alkibiades before being killed by Alkibiades' guards.