Silly name, but the guy had some serious moxy. Sisyphus was the son of King Aeolus who ruled almost as many kingdoms as he had vowels in his name. By all accounts, Sisyphus was a decent king in terms of keeping up a good economy and encouraging scientific growth, but as a person, he kind of sucked. Not only was he super greedy, but he was a serial killer to boot. He would lure people to his castle under the premise of Xenia (hospitality to guests) and then murder them while they slept.
He had a grudge against a man named Salmoneus, so to settle it, he seduced Sal’s daughter Tyro in hopes of impregnating her with children that would dethrone Sal. Sal figured out his plot and killed the babies.
Despite all his sins, somehow his most grievous error, the one that landed him in Tartarus, was tattling on Zeus for hiding the river god’s daughter.
While he was a horrible person, no one can deny the guy had guts. When Thanatos (or in versions of the myth that make NO sense, Hades) was chaining him up in Tartarus, Sisyphus tricked him into trying on the chains of death and trapped the god of death, and causing chaos when people would get mortally wounded and not die (anyone watch that episode of Twilight Zone, “One Night at Mercy?” Netflix it, it was awesome!).
Ares got bored with the never ending death free battles, so he saved the day by setting Thanatos free and returned Sisyphus to Tartarus. But Sisyphus wasn’t out of ideas yet. He whined to Persephone that his wife hadn’t buried him properly (she tossed his naked corpse out into the public square. I’d say she was making a statement, but really she was following Sisyphus’ instructions so he’d have a reason to go back), so she let him go back to the living realm with proper burial instructions. Once there, he decided not to leave and had to be dragged back to Tartarus by Hermes.
Because he’d thoroughly managed to piss off pretty much every god who had any say in his fate, Sisyphus was assigned an afterlife of pointless labor. He had to push a boulder up a steep hill, watch it roll right back down to where it started and repeat over and over again for all eternity. Interestingly enough, this punishment is alluded to in a good light in the Pilot myth of the Matched Trilogy. Sisyphus did get a brief break when Orpheus sang in Tartarus, but then it was right back to the grind stone.