Early in the first season of Final Space, former Infinity Guard Captain Quinn Ergon discovered she was leading a double life that even she didn't know about. A future version of herself named Nightfall had traveled back in time to play a role in the coming war, or at least in redirecting it. But fans were left wondering how Quinn would make the transition to Nightfall, with a major step in that direction being taken in Season 3.
Quinn made her first appearance in Season 1, Episode 1. In a failed attempt to woo her, Gary Goodspeed accidentally destroyed nearly 100 starships and a small, family-owned Mexican restaurant. It was this chance encounter that kicked off a string of events that would lead to Gary, Quinn and the rest of their allies fighting to stop Invictus from leaving Final Space and attacking their home dimension.
The story then picked up five years after that destructive first meeting between Quinn and Gary. They quite forcibly went their separate ways, with Gary serving a five-year prison sentence alone on a deep-space starship with KVN while Quinn continued her career in the Infinity Guard. Before Quinn and Gary were reunited, once again quite forcibly, Nightfall started to appear in the present time. And this version of Quinn had seen some things.
In her future, the war against Invictus was lost, and Gary was dead. Distraught, Quinn considered taking her own life but was stopped by the one good Titan named Bolo. He gave her a new mission, sending her through time to help shift the battle in a different direction. As is often the case with Final Space, there were a lot of questions about Nightfall, specifically relating to Quinn's transition to this alternate identity.
Quinn took the first step toward becoming Nightfall in Season 3, Episode 6. As she suffered from Final Space poisoning, HUE developed and installed an implant that would deal with the infection and keep her alive. This implant is an identifying trait of Nightfall's, one she also likely received due to Final Space poisoning at some point in her own history. Things took another step closer later when she put Nightfall's helmet on for the first time.
The unfortunate side of this is the fate of everything. Nightfall sacrificed herself to prevent Gary's death in Season 2, which happened in every other timeline. If Quinn inevitably becomes Nightfall, then his death may also be inevitable no matter what gets changed. Even worse, Invictus utterly annihilating their universe may also be inevitable, and that's a fate everyone is actively trying to avoid.
But there's another distinct possibility. The game may have been changed enough that victory is possible without requiring Gary's death. But for that to work, Quinn still has to become Nightfall, travel back in time and eventually sacrifice herself to save him. It's a sad fate, to say the least, but Quinn/Nightfall has made it clear that she would give anything to save Gary and reality itself. She made a bargain with fate, one she will have to honor if they are to win this war.