A cyber-weapon created by the Vau N'Akat and concealed aboard the USS Protostar, designed to spread to any Starfleet ship that makes contact and turn the fleet against itself.
The living construct is a self-propagating weapon that infects computer systems through communications. Once a ship is affected it fires on its neighbours, and the corruption passes on with each new signal. The Vau N'Akat of the future sent it back in time to destroy the Federation before first contact could tear their world apart.
It is revealed in the first season of Star Trek: Prodigy to be the true purpose of the Protostar. The young crew spend the later episodes trying to warn Starfleet without triggering it.
In the finale Supernova it is finally activated and Starfleet ships begin destroying one another. The crew stop it only by destroying the Protostar itself, at the cost of the Janeway hologram.