The son of Dia and Ixion, he loves Butes, but helped kidnap Helen, leaving her with Aethra.  
His sister Phisadaie was abducted by the Dioscuri and his son Polypoetes survived the Trojan War and buried Calchas.  He was present at the Calydonian boar hunt and once stole a head of cattle from Marathon.  The son of Dia and Ixion, he loves Butes, but helped kidnap Helen, leaving her with Aethra.  When he tried to secure Persephone from the underworld he was trapped when Hades feigned hospitality and bade him to sit in a throne from which he was never to emerge.  This Lapith king also riled up the centaurs at his wedding to Hippodamia.  FTP, identify this best friend of Theseus.
His sister Phisadaie was abducted by the Dioscuri and his son Polypoetes survived the Trojan War and buried Calchas.  
Pirithous or Peritho?s or Peirithous