User blog comment:Inquisitor dusk/tyranids vs flood/@comment-24.46.170.11-20130818153156

The Flood: they assimilate their food on all levels, whether multiorganisms or single celled organism. They would infect the Tyrannids, copy their way of going through space, and hunt down all Tyrannids. The Gravemind can tap into their Hive Minds, and win a psychological war. The gravemind's intellegence comes from previous experience from hosts, so it would draw information from Forerunners, Humans, Covenant, and any other race they've infected. The Tyrannids simply get wiped out by a superior race, and the Human Empire now has a greater foe to face, one they have no hope of winning against now that they have the entire Tyrannid population added to the origional population, and technologies from the races they've infected.