Just recently I have read a Halopaedia article regarding the Flood and I have to admit the concept is A-MA-ZING. I love it. Clearly deriving from the Biblical exceptional flood remarking the end of times or at least a threshold of certain aeon in mankind lasting, the Halo Flood - as a major test for the sentient species, primarily the humanity - implies that humanity has crossed certain border or came to the brink of another aeon, where the symbolical Godly mercy - symbolized with a rainbow - fails to apply any longer, forcing the sentient species to face another exceptionally destructive global occurence. The trick is, how to extrapolate the Eartly flood - dependent on Earthly atmospheric conditions of water circulation - to the cold bleak dust of cosmic space and bare rocks floating mainly around in it, especially if to maintain the militaristic focus of the franchise?
But I am always more delighted in conceptual reality than that of direct experience, therefore probably I am genuinely fascinated with the Flood only on the intellectual or noetic level, with direct encounter in the gameplay matrix probably up to leaving me disappointed, as everything that is brought down eventually becomes plain and obvious, devoid of magic or magneticism. That is why the Flood as something constantly "out there, lurking, watching, waiting for the right moment to strike" - when the stars align, so to say - resonates with me more than guns directed towards some indifferent biomass.
In a horror, it is actually better to leave the monsters untold rather than display them in full rubber glory.