In 2256 Discovery's sister ship USS Glenn was found with its crew dead after a spore drive jump, along with Klingon boarders and a large tardigrade-like creature.
The Glenn had been running the same mycelial propulsion research as Discovery and had made a long-distance jump before falling silent. A boarding party from Discovery, including Michael Burnham, found its crew killed by the jump and the bodies of Klingons who had come aboard.
The party also found a huge creature, later named Ripper, that had killed the Klingons. Burnham escaped, and Captain Lorca had the creature brought to Discovery, where it became the key to navigating the spore drive.