Can be used by a Vindicator or a Defiler and anything that has Havoc launchers, any who has been wounded but has not been removed from play must pass a toughness test or they have been infected by the toxins and other nasty things that the followers of Nurgle have placed inside the shells.
Use the stats of the weapon that is being used to deliver the shells. In the shooting players next turn the minis that failed the toughness test turn on their brethren and join the renegades. Wounded but not killed by a vindi or battlecannon shell?
So this will only affect terminators and high T models? I bit dodgy. i would make it an option for Havoc launchers OR, if it is going to be fired by the vindi or battlecannon, make it so it only hits at half normal strength and +2 AP for the cannon (to show that it contains the virus, not explosive) I would say, fluff wise, that the shells contain some sort of virus that sends the victims insane and they attack the their fellows (controlled by the player who fired the shell obviously).
The model does not count towards KP for the chaos player if killed. Maybe have it confer a USR like Rage to show just how mental the virus makes them. Dont bother changing the targets stats as it will get too complicated. Example: The Vindi fires the virus shell at a unit of vanilla marines (half strength so its a S5 AP 4 large blast). Wounds on a 3+. Wounds 5 marines, they all make their armour saves. Those 5 marines fail a Toughness test and are now controlled by the Chaos player and gain the Rage USR (meaning they HAVE to charge the unit they just left in the assault phase). I wouldn't make this a permanent change either. Maybe say the virus wears off after a turn or a successful LD test next turn?