Kastechia
All native carbon-based life has been converted to Kastechian spores.
Kastechia was a habitable world as recently as half a million years BCE. Since then, spores have consumed virtually all carbon on the planet, leaving an increasingly-tenuous atmosphere of nitrogen and some hydrogen.
Kastechia was discovered in TE 105 by an automated probe searching for habitable planets. Readings were odd enough to trigger an investigative team. When a landing party fell victim to the plague, the planet was closed and several more scientific teams were dispatched to determine the source and nature of the danger.