Plague Star is a recurring event within the Plains of Eidolon.
1.) Locate the Thrax Toxin in the cave waypointed on the Plains of Eidolon.
2.) Carry the toxin to the waypointed Toxin Mixer, and defend it. During this time, you can add Eidolon Phylaxes and Infested Catalysts to the toxin. Each Phylaxis used will add 250 standing to the final reward and will make the enemy’s level increase to 30 and make them spawn more frequently. Each Catalyst used will spawn a Hemocyte in the last phase of the bounty. A detailed chart can be found here. Up to 4 of each can be added during one run. You need to craft both these items in the foundry and have them equipped in your Gearwheel before starting the mission. The blueprint for Eidolon Phylaxis is purchasable from the Operational Supply Syndicate, which is available during the event, while the blueprint for the Infested Catalyst is available from Clan Bio Lab Research.
3.) Defend the Drone by escorting it to the Infested Boil.
4.) After the drone reaches the Infested Boil, infested enemies will spawn; you need to kill them to progress, with a percentage gauge on the left showing your total progress, and at certain points a Hemocyte will appear (Minimum of 1, Maximum of 4, based on number of catalysts used in the toxin mixer phase). These bosses are mechanically similar to the Lephantis boss fight found in Magnacidium, Deimos. Once the gauge reaches 100%, the bounty is complete and you can return to Cetus.