Children of Morta

All you need to do is find a map in one of the dungeon. The location is random like most things in the game. For me it was world 2-3 right before a fight with the formal boss of this dungeon. After short cutscenes there is another with old father. You have already met him in this dungeon with son and did some extra work for them.

In the second gate/world - Barahut - you have the chance to run into a series of quests related to the old man stargazer. There are multiple events you must complete before unlocking this:
- The first event, you will simply meet and talk to the stargazer and his son. (They have a wagon and talk about their observatory being guarded by a monster, but there is nothing for you to do here yet other than speak to them.)
- The second event, you come upon their observatory and will have to defeat a mini-boss.
- The third event, you have to do an escort mission with the son where you fight off enemies and keep him alive.
- The final event, you will discover aftermath of an encounter they had (nothing to do here besides interact to observe/comment on the scene).

I think only one event will spawn per run, so you will have to go through Barahut dungeons at least four times to complete the full sequence. After your run where you discover the aftermath, you will get a cutscene back at the house of the stargazer stopping by and the achievement will unlock.