Reclaim the Once Known
Our mission was simple, but it required us to be a little more underhanded than we liked to be. First, we purified the water on the planet. We took out all of the junk that the pesky dominant lifeform put in there. Much of the aquatic life was grateful for that. Then we destroyed most of the dominant lifeform's structures. It would have been a pity had they not been so malevolent in their structures. Made of the remains of other lifeforms, cruelly enough. With that destruction came a new creation - or well, a creation returning to its truest forms. It seems like the dominant lifeform should not have been the dominant one after all. But they have now paid their price.