What if the UNSC obtained ancient humanity's Lord of Admirals' ship with its Flood-destroying weapon?
Short answerThe requested vessel is not firmly identified; the later answer concerns Mantle's Approach and predicts overwhelming power if the UNSC can activate and control it.
NotesThe requested vessel is not securely identified: the Lord of Admirals is first clarified as a person, and the conversation then turns to the Didact's Mantle's Approach from Halo 4. That ship is described as overpowering Earth's defenses and using the Composer against humanity. Its enormous complement includes specialized personnel and more than a billion constructs, prompting disagreement over how much human crewing would actually be needed. Extensive automation could reduce the requirement to a compatible commander, but activation remains a condition of the scenario. If someone can turn it on and control it, it is expected to provide an overwhelming military advantage.