CREDIT GOES TO anatwick18 WHO DESCRIBED THIS IN A YOUTUBE COMMENT
SPOILER!!! The ‘ancient enemy’ is either the flood outbreak on Halo, humans, or something else. Halo: Cyrptum actually gives a TON of answers to this. Humans actually had a civilation about 110,000 years ago, and about forerunner level tech. The flood appeared, and humans managed to beat them back and find a cure/way to defeat them. They had to find new planets to regrow, so they went to forerunner territory. Forerunners got pissed, beat humans after a long war, and devolved them.
The Flood return, wipe forerunners out. The Prometheans mentioned at the end of the video are the elite warriors of the forerunners. Their leader was the Didact, mentioned in the Halo 3 terminals. He led the war against the humans, and was good at his job. He was married to the Librarian, which is why Spark mentions that she wouldn’t be there to temper him. He was the guy mentioned at the end of the video.
Here is the terminal that this relates to.
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