>>5667791In theory, they are a "Psycho Rangers" type squadron of elite Cobra henchmen, (loosely) modeled after the founding 13 GI Joe members who were recruited from the ranks of the various Vipers, Trooper divisions alongside other new characters.
In practice? They are what
>>5668266 said. They are Cobra Commander's personal wetworks team who are sent on missions where Cobra Commander wants people dead. And by dead, I mean people DIE when the Plague Troopers are deployed.
They are led by Blackout (evil brother to a new Joe introduced during Valor Vs Venom) and include Munitia (basically an evil counterpart to Scarlet, trained by Firefly alongside Blackout as his proteges), Guillotine (a former Snow Serpent/Eel), Aleph (a high ranking Night Creeper who was excommunicated), Nick Bailey (a generic black American soldier who CC personally recruited into the Plague Troopers for unknown reasons), the Interrogator, and a bunch of scrubs from the Saw Vipers, Sand Vipers, AVACs, Crimson Guard.
Rip It (HISS tank driver) and Sky Creeper (a Cobra Air Commando) were offered membership and went on the group's first mission (massacre an entire hospital, children included, in order to kidnap Baroness and Destro's kid) but were both executed by Cobra Commander because they had hesitated during the mission and openly questioned why they should kill innocent babies.
Ironically, while they have a Saw Viper on their roster (Gallows), it's not the infamous Saw Viper who killed a dozen GI Joe members in the early 90s (he became Overkill, a move that I'm sure Devil's Due regretted years later when they came up with the Plague Troop concept and couldn't use the guy for it).