The Stygian Ice Whistle is made of ice from the River Styx, as such, it feels unnaturally cold while on the hand. It is very hard to craft and will only melt when used. Stygian ice can only be used once before it shatters into a 'squillion pieces' as Quintus a.k.a. Daedalus told Percy.
Before leaving on his quest, Quintus (who is Daedalus) gave Percy Jackson a dog whistle made from Stygian Ice as a gift in case he needed help. The whistle was specifically designed to call the hellhound Mrs. O'Leary to whoever blew into the whistle. While untrusting of Quintus, he accepted the whistle and placed it in his pocket, although promising himself that he wouldn't use it. Quintus warned Percy that the whistle can only be used once and would shatter to pieces once used. He was also unsure if Mrs. O'Leary would be able to hear the whistle in the labyrinth.
Percy on several occasions mentioned the whistle getting colder, normally whenever Nico di Angelo would call ghosts from the Underworld to talk with, regardless of if Percy as there or seeing it in an Iris Message. It also began to grow colder when Percy himself was close to dying, such as when he caused an eruption on Mount Saint Helens in order to escape a pack of Telekhines. After the eruption, the Stygian Ice whistle somehow followed Percy to Ogygia.
Percy was forced to use the whistle in Antaeus' Arena after walking into a trap that almost resulted in the deaths of Annabeth, Rachel, and Ethan. Percy blew into it, causing Mrs. O'Leary to appear in the arena, attacking the monster filled stadium. However, the whistle shattered right after it was used.
- The whistle can call Mrs. O'Leary from almost anywhere.
- Just like Percy's sword, Anaklusmos, the whistle returns to the owner if lost (as demonstrated when Percy was sent to Ogygia, and the whistle was still in his pocket).
- The whistle does not melt unless it is used. When it is used it shatters.
- When used, there is no sound (at least not of a pitch that humans can hear), so it might work like a dog whistle. And because it was made from ice from the Styx, it may only call to Underworld Beasts.
- The whistle will grow colder the closer the person holding it is to death (whether it be someone calling the dead or a person close to dying).