Sisyphus was the founder and first king of Ephyra (now Corinth.) He was in general an all-around greedy, murderous, lying, deceitful, nasty person, who ticked off the gods many times and in many ways.
Finally, Sisyphus crossed a line when he ratted out Zeus and the gods decided it was time for him to be punished. Zeus sent Thanatos (Death himself) to chain King Sisyphus down below in Tartarus. (Some versions substitute in Hades, god of the Underworld). King Sisyphus slyly asked Thanatos to demonstrate how the chains worked, and Sisyphus seized the opportunity and chained Thanatos instead. Once Thanatos was bound by the strong chains, no one could die on earth. This caused a general uproar. Especially mad was Ares, god of war, who was annoyed that his battles weren't much fun anymore because his opponents would not die. The exasperated Ares freed Thanatos and turned King Sisyphus over to Death.
Sisyphus then concocted a plan with his wife for her not to give him a proper burial or put a coin under his tongue to pay for his passage to the underworld across the river Styx. Then he tricked Persephone, Queen of the Underworld, to send him back to earth to punish his wife for not giving him a proper burial. He lived for a good amount of time happily with his wife afterward until he finally died of old age.
In punishment for his trickery, he was sentenced to roll a giant boulder up to the top of a hill, but Zeus enchanted the boulder and, when it finally reached the top, would always roll back down to the bottom. Then Sisyphus would have to start over - a punishment of an eternity of worthless hard laor. To this day, seemingly endless tasks that are laborious, pointless, and futile are now described as Sisyphean.