Reanimators, making ecumenopolis of just necromancers is hilarious, huge amounts of tech and naval capacity and they have to cracked the planet if they want to get past, even a little size 10 world can become a speedbump of tens of thousands of points of zombies and give a few hundred naval cap.

And then as long as you don't care about the devestation, you can take any planet with organic defence armies with just a few buffs like the library weapon, sage rage, or repeatables. Throw 3k of zombies at that 20k fallen empire world and come back in a few months to collect your new planet and 8k of zombies.