A super-mycelial reactor was a mirror universe technology utilizing the mycelial network. It powered the Terran Empire flagship, the ISS Charon. While not sustainable, it allowed the powering of the city-sized ship and its weapons, which were powerful enough to destroy a planet.
The reactor differed from spore drive in that it pulled energy directly out of the mycelial network. Its reactor core, a large central "energy orb" containing mycelial spores packed together, blocked the flow of the network to harness exotic energy. The core also gave off a massive energy signature, and its spores created hyper-gravitational and magnetic fields.
While the energy orb was located outside of the ship's superstructure (though at the heart of it), it was protected by a containment field, which was controlled directly from the ship's throne room.
The super-mycelial reactor was unsustainable and in fact poisoned the network, spreading infection inside it. It was theorized that the Terrans were aware of this, but did not care. While trapped in the mirror universe, the USS Discovery determined that its reactor might eventually destroy the entire network, which would result in life ceasing to exist across the multiverse.
The threat was averted when Discovery's acting captain Saru flew his ship through the bowels of the Charon and destroyed its reactor. The resulting explosion, consisting of pure mycelial energy, also produced a large mycelial shockwave. It powered Discovery's spore drive and allowed the ship to navigate back to its home universe. (DIS: "What's Past Is Prologue")