Stygian Ice is a rare material that can only be made in the Underworld's River Styx as it is cooled. 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 it to form a whistle, which can be used to summon hellhounds. However, it will shatter after only one use. Stygian Ice is similar to Stygian Iron, as both are dipped in the River Styx during their forging process.
Before leaving on his quest, Quintus 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 distrusting 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. 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 Chase, Rachel Elizabeth Dare, and Ethan Nakamura. Percy blew into it, causing the hellhound Mrs. O'Leary to appear in the arena, attacking the monster-filled stadium. However, the whistle shattered right after it was used.