Theseus and Pirithous were described as ‘’blood brothers’’, in Greek mythology. When Pirithous wife (Hippodamia) died, Pirithous came to Athens and found that Theseus' wife Phaedra was also dead. Pirithous and Theseus, who had grown older, decided to marry daughters of Zeus.
Theseus chose the young girl Helen (yes that Helen of Troy) and together they kidnapped her, intending to keep her until she was old enough to marry Theseus. Pirithous chose Persephone, the queen of the underworld. Pirithous and Theseus left Helen with Theseus' mother Aethra and traveled to the underworld domain of Persephone and her husband Plouton/Hades. As they wandered through the outskirts of underworld, they sat down to rest on the rock. Soon they found themselves unable to stand up from the rock and saw the Erinyes appear before them. Pirithous and Theseus were stretched out and tortured by the Erinyes.
According to some version, Hades, the king of the underworld, cheerfully welcomed Pirithous and Theseus in the underworld and asked them to take a sit. When they sat on the chair, they saw themselves grow fast to the chair of forgetfulness, being held there either because the rock grew to their flesh or by coils of snakes.
Later Hercules found them when he came to underworld to fetch Hades' three-headed hound Cerberus (Hercules twelfth labour). Hercules freed Theseus, but when he tried to free Pirithous, the earth shook. As Pirithous had committed greater crime, as he wanted to marry the queen of the underworld. So Pirithous was stuck forever in the underworld.