The Catalyst is a main quest in Fallout 76, introduced in the Steel Reign update. It is the final quest in the main Steel Reign quest line. Completing this quest unlocks the Steel Brethren trophy.
Side with Paladin Rahmani
If you decide to side with Paladin Rahmani, Knight Shin will refuse to serve the Fort Atlas anymore that threaten to go and inform the elders about the decision. At this point, you can choose to attack Knight Shin or let him go.
Go back to Fort Atlas for the ending of the Brotherhood of Steel storyline. Paladin Rahmani will inform everyone that Knight Shin has left the Appalachian Brotherhood and confer you with the title of Knight-Errant. At this point, you will have the option to flirt with Paladin Rahmani and Scribe Valdez. You can also talk to Russel Dorsey and Hewsen. After you have done talking to everyone, speak with Rahmani to complete the mission.
Paladin Rahmani announcment
“Mutant threat has been cornered and removed. But it came with an unfortunate consequence. I’m sure you all know that relations between Shin and I have been strained for some time now. I’m afraid my leaving out his title is no mistake. Shin and his followers have decided to part ways with the Appalachian Brotherhood. I had every wish to worth together with shin on my mission and still do. But he is no longer open to discussion. His loyalty to the Elders is unshakable. He would rescind his Oath before besmirching the Brotherhood’s name. Should he ever open his mind, I will be ready to listen.“
Side with Knight Shin
If you decide to side with Knight Shin, Paladin Rahmani will disagree with Shin’s decision and decided to leave the brotherhood to start anew elsewhere. At this point, you can choose to attack Paladin Rahmani or let him go. You will get options to kill scientists yourself or let Knight Shin do the task.
Go back to Fort Atlas for the ending of the Brotherhood of Steel storyline. Knight Shin will make an announcement on Paladin Rahmani and also on other things. You will confer with the title of Knight-Errant.
Knight Shin announcement (after killing Paladin Rahmani)
If you have killed Paladin Rahmani then Knight Shin will inform everyone that Paladin Rahmani had given her life to defend criminals and she was following her own misguided moral principles. The full transcript is below.
It pains me to say that Rahmani gave her life to defend these criminals. Her last act was not as a Brotherhood Paladin, but as a person following her misguided moral principles. Rahmani put her personal morals above the cornerstone of duty time and time again. Her irresponsibility threatens to shake apart the foundation of order that allows us to be an effective force of action. But however misguided her decisions were, Rahmani always acted with the intent to help humanity. If we were to retain anything from her leadership, let it be that.
This video is from Tof Gaming
Knight Shin announcement (without killing Paladin Rahmani)
If you have left Paladin Rahmani then Knight Shin will inform everyone that Paladin Rahmani betrayed the brotherhood code and she refuses to punish the criminals. She decided to leave the Appalachian Brotherhood and choose her own path in exile. The full transcript is below.
“The mutant threat has been removed at its source. You may have noticed that Rahmani is not the one making this announcement. This is because her value and those of the Brotherhood could no longer be reconciled. Rahmani betrayed the Brotherhood code when she cut us off from the Elders. Today her misconduct took the form of refusing to punish the criminal behind the mutant threat, and the countless deaths it caused. She couldn’t bear the weight of justice on her shoulders and thus chose to walk her own path, in exile. Rahmani put her personal morals above the cornerstone of duty time and time again. Her irresponsibility threatens to shake apart the foundation of order that allows us to be an effective force of action. But however misguided her decisions were, Rahmani always acted with the intent to help humanity.”
This video is from Game Guides Channel