So the full question is:
What if…A fully crewed and equipped Executor-class Star Dreadnought arrived in UNSC controlled space prior to the fall of Reach, with no means of returning home?
How would they react to encountering such a primitive tribe of humans?
What would be the reaction of the UNSC?
Would the SSD Admiral even reveal his presence to the UNSC?
Would UNSC diplomats attempt to negotiate a pact, or draw them into the Covenant-Human war?
If so, how well would the Covenant Empire fare against a fully armed and equipped Executor-class Star Dreadnought?
Or, would they just bugger off to another part of the galaxy and ignore everyone all together?