The Brethren Moons have access to their Markers and Necromorphs.
The Gravemind has access to their Flood.
A few things for those possibly not familiar with Brethren Moons.
- They are mobile and can "naturally" move at FTL speeds.
- They are able to produce Markers which have the capability of unlimited or near unlimited amounts of energy.
- They are telepathic and can cause hallucinations even on people who are not infected.
- There are at least seven known Moons in existence throughout the galaxy as seen in the end of Awakened. They are also able to communicate which each other over vast distances.
- They can create more Moons using the method of Convergence.
Nobody else is involved. No Isaac Clarke, no Master Chief, no Earthgov and no UNSC. It’s just Brethern Moons duking it out with Gravemind, mono e mono (uh, I think that’s how you spell it).
Now, my vote in this case would go with the Moons for the numerous reasons above. Remember I’m referring to UNSC (game) era Gravemind and not the god-like Flood of the Forerunner era with their Keyminds and whatnot.