Ok, I can already tell that lore librarians in the comments are saying "But both universes have The same common ancestor universe, which is us, so they can't exist together!" Well let me set the scenario up for you: In an alternate timeline of the Halo universe, The Flood won the war against the Forerunners, and the Halo rings were never fired. The infection spreads through the Galaxy until every Organism, biological or not, is consumed. They then become extra-galactic and start to spread to other galaxies. Now lets just say that they found a species in, lets just say Andromeda, found a way to travel between universes. The Flood, always looking for new places to spread, consumes this species and gains the ability to spread [i]through universes[/i]. As you can tell, this is very, very bad, as the Flood will now search for infinity for new universes to infect. Eventually, the flood just so happens to come across the Destiny Universe at around the time of Forsaken. They start, for some reason, in Sol, and Billions of Flood forms begin to show up on Triton. But, for some reason, the gateway was closed early and only a few flood forms got through. For numbers, lets just say this is how many forms got through: 500,000 Combat forms 1 million infector forms 1 Proto-Gravemind In comparison to the rest of the hive mind, this doesn't seem like a lot, right? And the Flood don't know anything about this universe. But, luckily for the Flood, A fallen ketch belonging to the remnants of the House of Kings enters the moon's orbit.. [i]And they thought their problems were over...[/i] So the ketch is consumed and the Flood now know everything the Kings know. Plus, the ketch is now in possession of the flood, so they can get off-world. Now, lore master mine, what happens now? Get as creative with this as you want!