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By pledging to the defenders of earth, by pledging to the followers of war and by pledging to the seekers of exodus. I am 100% confident these pertain to the 3 factions: New Monarchy, FWC and Dead Orbit. You 'pledge' to them, and 'followers of war' & 'seekers of exodus' fit with FWC and DO exactly. 'Defenders of Earth' you have to dig a little deeper to pair it with NM, but Hideo's whole thing is to start a new Consensus, thereby transferring power from the Vanguard, whose primary function it is to defend Earth and The Last City. (Also it just stands to reason if the other two are "matched", this is the only faction left!) Lastly, these are the 3 exotic weapons you are gifted from Asher Mir, by playing through the original D2 story, which is not coincidence.
By pledging to the defenders of Earth (New Monarchy)
Sweet Business
By pledging to the followers of War (FWC)
Sunshot
By pledging to the seekers of exodus (Dead Orbit)
Graviton Lance
Dead Orbit it is.