The dreadful Assimilation Plague. This Bio weapon is pretty high up the tech tree due to the effects of it on a planet. The Assimilation Plague can be used by by any ship that has a bio weapon section. The smaller the ship the less missile. These weapon must be used in mass. Meaning that you have to shot a bunch of this missiles to have effect. What it does is convert the Imperial Population/Civilian population to you and war between the infected and the non-infected break out and the side with the most usually wins. Thus is why you should use as many missiles as possible to get the effect. It can take up to 2 -3 turns until the planet converts to you.
TIP- When this plague is in process attacking the planet with other weapons can kill your infected civilians and thiers as well.