After KF2 showed the destructive power of the ZEDs and the mercenary groups combating unruly outbreaks gave the company enough goodwill in the eyes of the public, Horzine has since been producing literally millions of ZEDs per year per ZED type to supply to governments worldwide as a means to replace conventional armies. This has let them become the world's largest and most powerful corporation, at this point owning world governments either by convenience, by money, or by the sheer threat of an "accidental" outbreak in any unruly country.
Kevin's granddaughter and heir to a portion of the Horzine fortune has used her position and resources to fund a group called Nightfall and gather people from around the world as a form of resistance and off the records investigation unit to catch any would be mad scientists and end their plans, which is what these first mission stories are all about with the Chimera Project. I couldn't find anything Nightfall being or not a secret from the game's lore, so maybe that'll be expanded in the next season.