The Hedron is an extradimensional being originating from Slidescape-36 which acted as a conduit of Polaris's resonance. Discovered and studied by Casper Darling, it was used to develop Hedron Resonance Amplifiers (HRAs) which would later protect the Bureau from the invasion of the Hiss, a hostile resonance.
The Hedron entity was discovered in Slidescape-36 during an expedition led by Casper Darling of the Federal Bureau of Control, using the Slide Projector Object of Power discovered during the Ordinary AWE. Darling identified it as a powerful source of resonance and, in spite of casualties resulting from dimensional resonance in Slidescape-36, had the Hedron relocated to Earth and placed within a massive chamber in the Dimensional Research area of the Research Sector. There, the Hedron was extensively studied by Darling, who identified it as a benign and intelligent source of resonance.
According to Jesse Faden, the Hedron acted as a conduit or amplifier for Polaris, a broader resonance-based entity. Aware of the impending Hiss invasion, the Hedron relayed to Casper Darling the threat, prompting him to create Hedron Resonance Amplifiers (HRAs) to protect Bureau agents from hostile resonance. Director Zachariah Trench, who had fallen under the Hiss's influence while on one of the Slidescape-36 expeditions, used another slide to unleash the Hiss upon the Oldest House, with only the agents wearing HRAs surviving. At the same time, the Hedron called out to Jesse Faden to reach the Oldest House and protect the world from the Hiss. Jesse became the Bureau's new Director after Trench's death and helped to repel the Hiss, but Hedron was destroyed by the Hiss, nearly resulting in the Bureau's destruction; however, as Jesse became Polaris' new conduit, the Bureau was saved.
While Jesse Faden initially mistook Hedron for Polaris, they are distinguished as two separate entities. Hedron acted as something of an amplifier or source of Polaris's resonance, distinct from Polaris herself, and acted according to its own agency. According to Casper Darling, the Hedron was benign and sought to protect the Bureau and the Earth from the Hiss threat, which is why it called upon Jesse Faden.