So these are found within the borders of fallen empires. Specifically Militant Isolationist empires. So if your game doesn't have one you can't get this achievement. Send your science ships into their area and start scanning planets. There will always be three within their borders. When you find one it will show up as a research project. Have your scientist complete it and you'll get the achievement. You'll get an admiral, some of their ships, or a void cloud.