The Covenant (Halo) vs The Grand Imperium of Man (Warhammer 40k)

[Tiki Snakes]

I'm not entirely sure how directly relevant the flood even were in the downfall of the covenant, except coincidentally and as part of the reason that the prophets turned on the Elites.

edit - The downfall of the covenant isn't about the elites beating the loyalists. The covenant falls, or rather is broken, when the Sangheli learn about the Prophets betrayal and their new favourites the brutes massacring the elites on earth. (Finding out the truth behind the halo rings and the great journey was a catalyst of this and the cherry on the cake, because they seem to have fallen out of favour at least partly because of the sangheli rebels you take on as the arbiter who seem to suspect the truth of the great journey).

Once Sangeheli turn on Brute/Prophet, the Covenant ceases to meaningfully exist and the only way forward is down. It also almost entirely happens because of the Prophets attitudes to those underneath them and the basic mismatch with the fanatical but essentially honourable Elites. (Despite being as dumb as bricks, the unquestioning and ammoral brutes make much better servants for the Prophets in that regard).