The Future Achievement in The Talos Principle II

This achievement is related to a secret ending you can achieve, but to do so required quite a bit of work:
 

1. You'll first need to collect all 24 stars. Refer to each area's achievement above for a video guide showing how to complete the star puzzles in each area
2. Second, during Act 5, head to the gold star on your compass at the Megastructure landing area and interact with the wall. This will input all of your gold stars, show you a vision, and unlock The Edge of Forever (60G) 
3. You now have access to all golden gate puzzles. There is one per area, so take the shuttle to each area and complete each one's single golden gate puzzle. These are now marked on your compass
4. Return the Megastructure landing area, and head to the golden gate marked on your compass. Talk to the person there and complete the challenge presented to you (this is very simple and straightforward, and there's no way to fail it). Completing it unlocks Ab Aeterno (30G) 
5. Now proceed to the VTOL to enter the Megastructure for the last time. Reach Athena, proceeding through the dialogue with the only available dialogue choice until you get to a point where you have three dialogue choices to choose from. Each one results in a different ending. Choose the top option, "I want to know the truth," to unlock this achievement.

Note that this achievement isn't missable, because the game creates an autosave before entering the Megastructure. So, if you don't get this ending but still beat the game, when you continue your save file, you'll be back at the Megastructure landing, free to mop up whatever puzzles you missed or steps you didn't complete for this achievement.