Stygian Ice

Stygian Ice is a magical type of ice that is made from the water of the River Styx.

Description
Stygian Ice is a rare material that can only be made in the Underworld, as it is cooled in the River Styx. Stygian Ice can be forged into different shapes to be used for different purposes, but it is very hard to craft and is also very fragile. Despite its' fragility, it will not melt like normal ice. However, when the holder of Stygian Ice is close to death, the ice will become colder until it feels like it is burning, but from cold instead of heat.

The only uses for Stygian Ice shown in the series is to use to it form a whistle, when can be used to summon hellhounds. However, it will shatter after only one use. Stygian Ice is related to Stygian Iron, as both are dipped in the River Styx during their forging process.

The Battle of the Labyrinth
Before leaving on his quest, Quintus (a.k.a. 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. Quintus warned Percy that the whistle can only be used once and would shatter to pieces once used.

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.

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.