Orpheus

Orpheus (Ορφέας in Ancient Greek) was the son of Apollo and Calliope (the eldest of the nine Muses).

History
Orpheus was a very gifted musician trained by Apollo himself.His music was so beautiful and sweet that it charmed many wild beasts, made trees sob and even created an entrance to the Underworld itself, by singing his way right into Hades' palace to rescue his wife Eurydice after she was bitten by a snake on their wedding day. Hades allowed him to take his wife back, on the condition that he would walk in front and not look behind him as he led her back to the upper world as long as they proceed to the borderland between the living and the dead. However, he couldn't resist the temptation and looked back just before they left the Underworld, losing his wife forever. Orpheus was later killed by the Maenads on Dionysus' orders and his soul was reunited with Eurydice.



The Lightning Thief
It was mentioned by Grover when talking about heroes that entered to the Underworld.

The Last Olympian
There is a Door of Orpheus located in Manhattan's Central Park that is named after him (as he was the one that created it). Nico di Angelo, a son of Hades, and Percy ask Grover to open the door using his reed pipes, as it can be only opened by music. Considering Percy's comments on Grover's reed pipes music, either any music, good or bad, may open the doors, or Grover's music has improved.