It could be about a lost star in the context of 'The 100' universe. Maybe it's a story of a character who is like a lost star, feeling alone or out of place among the events and people in the 'The 100' world.
Well, the plot might involve a lost character finding their way back or uncovering some big secret that changes everything in the 'The 100' world. There could also be unexpected alliances or conflicts.
Well, 'the lost son star wars fanfic' might focus on a son who has been separated from his family in the Star Wars setting. It could be that he's lost in terms of his allegiance, not sure whether to side with the Jedi or the Sith. He might be on a quest to discover his roots and in the process, face many challenges like battles, moral dilemmas, and dealing with different alien species.
Since it's a 'lost son' story, the main character is likely to be the son himself. But without reading the fanfic, it's hard to say exactly who he is or what his backstory is.
I'm not that familiar with specific 'The 100' fanfics similar to this one. But you could try looking on fanfic platforms and searching for 'The 100' fanfics with themes like lost characters or hope in a desolate world.
Since it's a fanfic about lost primarchs, the main characters are likely to be the lost primarchs themselves. However, it could also include other characters who are affected by their loss or the mystery surrounding them, like the remaining primarchs or some of the key military leaders in the story's universe.
It might be related to certain quests. For example, players could be tasked with finding the 'lost ark star of our story' which would lead them to new areas and rewards.
You could try using search engines and entering specific keywords related to the fanfic. Also, check relevant fanfiction archives or forums where it might have been posted.