A Federation dilithium mining colony whose rescue from a Klingon attack was the first major success of Discovery's spore drive.
Corvan II is a dilithium mining colony that comes under Klingon attack in The Butcher's Knife Cares Not for the Lamb's Cry. Starfleet orders the USS Discovery to defend it, but the distance makes Paul Stamets reluctant to attempt such a long jump, and an early attempt nearly drops the ship into a star.
Once the giant tardigrade later called Ripper is connected to the spore drive in Engineering, Discovery makes the jump successfully and arrives in time to save the colony. The rescue convinces Gabriel Lorca that the drive can change the course of the Klingon War.