I'm telling you the myth anyways.
So once upon a time in ancient Athens Theseus had a friend named Pirithous, who like 10% of mythological Greece was a child of Zeus. So Pirithous' wife dies and he decides to visit his friend Theseus. Once he gets there he finds out that his wife is ALSO dead. This is a drag, but it presents a unique opportunity. Finding a new wife.
So these absolute geniuses get scheming and Theseus decides the wife for him is Helen (of Troy fame) who was only 12 at the time. But Pirithous helps him kidnap her anyways.
Their brilliant plan to deal with her being 12 is to just keep her around until she's of legal marriage age. So that's... fun. (she gets rescued later)
But after that it's Pirithous' turn and his brilliant idea is to marry Persephone. You know. The queen of the dead.
So instead of thinking, Theseus decides to help him with this brilliant idea. They make the treacherous journey down to the underworld and once they get there, Hades is there.
Hades offers them a seat at the table. And, once again, alarm bells do not go off in their heads because they sit down on the bench and the second they sit down, their butts fuse to the stone bench and Hades sends the furies to torment them.
Several months later Heracles shows up on his quest to borrow Cerberus and comes across Theseus and Pirithous still stuck to the bench. So they catch him up on their situation and he decides to try and free them.
So Heracles pulls up Theseus first but his ass and part of his thighs are left behind. Other than that he's mostly fine though so he goes home, presumably to get some ointment. Then Heracles goes to free Pirithous but when he tries to lift him up the ground shakes and Heracles takes that as a sign that Pirithous is meant to stay there.
And for all we know, Pirithous is still sitting on that bench. Getting periodically bothered by the furies right next to Thesesus' disembodied butt and thigh meat.