Suggested by Xeno Dimentio
This is a war between two scourges of their respective galaxies.
We have the Flood rom the Halo series and the Xenomorphs from the Alien franchise. The location is the Same as the Flood vs Metroids match. There are 5 million assorted Flood forms (without a Gravemind) at one pole, and 5 million Xenos of assorted types (including Queens) at the opposite pole.
Each has a steady supply of Human hosts to increase their forces.
Who wins this parasite war?