What do i need to do to get "There’s a Zombie on my Lawn" achievement?

[Kufesska]

I have necrophage-necroid empire, with total war CB i conquered all systems of plantoid empire. What i did wrong?

[Ragnarr Loðbrók]

https://www.trueachievements.com/a371389/theres-a-zombie-on-my-lawn-achievement

[Bored Peon]

I got mine observing a Pre-FTL hive empire during my Recent History run. When it wanted the system I said no. Since hive cannot be absorbed it was wiped out and the achievement popped up.

All you have to do is wipe out the civilization without blowing up their last planet with a Colosssus.

[Kufesska]

ohhh, maybe the problem was caused by pops that escaped the necrophage purge

[Mennan]

By "Conquer" did you completely remove the plant empire from the game? Or did you simply win a few colonies and now they have a few more in some far corner?

[Kufesska]

I completely removed them, all their planets and systems belong to me now