More than anything else, Covenant needed to explain what the heck its prequel Prometheus was all about. Seen by many as an unnecessary addition to a pretty solid trilogy about scary space monsters on a murder spree, Prometheus introduced us to the Engineers, an alien race who inexplicably catalyzed life on Earth. We don't know exactly why, and that's what the crew of the Prometheus sets out to discover.
However, once the android David actually finds the Engineers and all of the answers are within reach... he slimes them. An entire city of Engineers is destroyed by David's stock of evil black goo. We never learn who the Engineers are, why they do what they do, or where they came from. And thanks to David, we may never know.
