The thing goes this way. The Guardians of Time have created a random Multiverse and have transported Gallifrey and everything in Halo, ME and Starcraft to it. Soon, the Time Lords decide to meddle into the others' affairs, and so they ally against the Time Lords.
These are competent Time Lords. Rassilon, The Doctor and The Master are allowed, but Omega isn't by virtue of being a universe buster and an antimatter god. The Doctor's plot shields are deactivated in this match.
For Halo, everything is allowed. That includes the Precursors and the Forerunners. Same for Mass Effect, Reapers and Leviathans can go all out along with everything. In Starcraft, everything is allowed (including Hybrids, Primal Kerrigan and the Xel'naga).
How does this go?
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Um. You have trillions of Zerg,Flood and Forerunners to deal with. If the precursors are allowed then i see no reason for the Xel'naga not to be, we know just as little of them.