Pirithous was the hero left stuck on the Chair of Forgetfulness in the underworld.
Pirithous and his close friend Theseus descended into Hades after deciding that each would marry a daughter of Zeus. Theseus had helped seize Helen, while Pirithous chose Persephone, the wife of Hades. Their attempt to claim the underworld queen was treated as a serious violation, and Hades received them under the pretense of hospitality.
The pair sat on the Chair, or throne, of Forgetfulness, sometimes called the chairs of forgetfulness in plural. Serpents bound them to the seat, and in some accounts their bodies grew fast to it. The chair’s association with Lethe, the river of oblivion, explains its name and its connection with lost memory.
When Heracles later entered the underworld to capture Cerberus, he rescued Theseus. He attempted to free Pirithous as well, but the earth shook, so Pirithous remained behind. Some versions instead say that Cerberus killed him. Theseus is therefore a common mix-up: he was also trapped, but Pirithous was the hero left stuck.