Archonexus Ending
Basically uhh if you're way into a game and haven't gone for the other endings a faction will come to you and say "Hey here's a map fragment pick 5 colonists and animals and move here".
Once you do that your first colony will become an NPC colony, all of your research will be reset and you'll have to start again.
Once you get to the same "certain point" you were in last time you'll get another map fragment and have to move again to this archonexus shrine, battle some waves of enemies and then get a cool ending.
This reddit comment has a more detailed explanation about it but it looks like a BIG hassle so i'm marking this as "enhancement" and not putting it as necessary for 1.3.
It'd prob be just syncing the events from the first map fragment giver and the interactions with the archonexus.