What Halo story would you base it off?
I personally thinks that Ghosts Of Onyx would rock the final scene in the teleporter room would be crazy
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Contact Harvest would be the most realistic for an adaptation budget-wise, I think. It was a great story to-boot and would be a pretty fitting start by showing the origins of the Human-Covenant war since none of the games go into any detail about the events this covers (sans Halo Wars).