Final answer:
Tantalus is the sinner from Greco-Roman mythology who was eternally punished for an act of cannibalism by being placed in a situation where food and water were always just out of reach.
Step-by-step explanation:
The famous sinner from Greco-Roman mythology who was eternally punished for an act of cannibalism is Tantalus. In Greek mythology, Tantalus was condemned to the Underworld for a series of offenses against the gods, including the most infamous one where he killed his son, Pelops, and served him as a meal to the gods. This act of hubris and violation was met with eternal punishment; he was made to stand in a pool of water beneath a fruit tree with low branches, with the fruit ever eluding his grasp, and the water always receding before he could take a drink, hence the origin of the word 'tantalize'.