Blind Betrayal

Blind Betrayal is a Brotherhood of Steel main quest and achievement/trophy in Fallout 4.

Detailed walkthrough
After Liberty Reprimed is completed the Sole Survivor is ordered to report to the Prydwen. There, a furious Elder Maxson confronts the Survivor, demanding to know if they were aware Paladin Danse was a synth. He reveals that the network hack of the Institute's mainframe yielded indisputable proof Danse is not human, but is actually a missing construct known as unit designation M7-97. The Survivor is then given a choice of medium-difficulty persuasion checks to convince Maxson they did not know Danse's true nature. Alternatively they can suggest that Danse himself may not have known he was a synth.

The Survivor is then ordered to find Danse, who has gone AWOL, and execute him. They meet with Proctor Quinlan to figure out a way to track Danse down, but their deliberations are interrupted by Scribe Haylen. Haylen pulls the Survivor aside and makes a plea for mercy on Danse's behalf, desperate to save him even if he is a synth. She then directs the Survivor to the recon team's previously declared fallback location: Listening Post Bravo.

After dispatching the security measures (two exterior turrets, one exterior protectron, and three additional protectrons down the elevator), the Survivor locates Danse inside the bunker. There they must decide how to proceed.

Plan A: Carry Out the Execution
The Survivor may execute Danse with or without speaking to him. Alternatively one can let him commit suicide. On his body they will find a synth component and Danse's holotags. Collect the tags, return to the Prydwen, and inform Elder Maxson the deed is done.

If the Survivor enters the bunker and is seen by Danse, then leaves the bunker, Danse will commit suicide.

Plan B: Allow Danse to Live
The Survivor may refuse to execute Danse. He will protest, stating synths are abominations and that he must be made an example of. Passing a medium-level persuasion check will convince him he is no threat to humanity and should flee the Commonwealth. Danse hands over his holotags to appease Maxson, then asks the Survivor to meet him on the surface to say their farewells.

Outside the bunker Danse and the Survivor are met with an unwelcome surprise: Elder Maxson, having suspected the Survivor would prove insubordinate, has followed them to the bunker. Outraged, he reiterates his orders and demands the synth's death. From here the Survivor has three options:
 * Concede to Maxson and kill Danse. Danse will submit peacefully.
 * Fail to convince Maxson to spare Danse OR request he perform the execution himself. Maxson will kill Danse.
 * Persuade Maxson to let Danse live. Maxson will reluctantly agree, but will exile Danse from the Brotherhood. As a known synth Danse will be fired upon by Brotherhood soldiers.

Whatever decision is made, return to the Prydwen. Maxson will promote the Survivor to the rank of Paladin in addition to granting them all of Paladin Danse's belongings, including his living quarters (the room marked "Prydwen Quarters" which is locked before this quest) and his personal set of T-60d power armor.

Companion reactions
Depending on which companion is accompanying the Sole Survivor, they will give out some unique dialogue on the situation.
 * Cait encourages killing Danse as she believes he has been lying to the Sole Survivor about his identity. She'll also wonder if anyone can feel safe with the Institute "churnin' out copies of people."
 * Codsworth will defend Danse saying that they don't easily jump to kill people for short reasons.
 * Curie will object to the idea of killing a loyal soldier and friend like Danse, saying it shouldn't matter if he is a synth.
 * Deacon says that though Danse has a "stick up his ass" he's still their pal. He later remarks that "This is what the Brotherhood's really about."
 * Hancock tells Maxson that if he wants Danse to die, then he should do it himself. He later calls the Brotherhood disloyal for betraying Danse after he spent his whole life with them.
 * Nick defends Danse saying that he isn't the guilty one in the situation and Maxson's orders are not justice. He later wonders how Danse feels about himself after finding out that he has been a synth his whole life.
 * Piper calls Maxson a lunatic and says Danse hasn't even done anything wrong. Later she states she can't believe Danse is a synth, "Except, you know, I can because I've heard him speak."
 * Preston argues that Maxson's orders don't make sense and Danse shouldn't be killed just because he "might" be a synth. He later believes no one is safe if the Institute is able to infiltrate the Brotherhood of Steel.
 * MacCready condones killing Danse for being a synth alone, and also for fear of becoming a target of the Brotherhood if they don't. He later claims that he isn't surprised that Danse is a synth due to his lack of emotion. Interestingly, MacCready will express regret after Danse is killed. If Danse is saved MacCready will comment that he appreciates that the Sole Survivor is a loyal friend.
 * Although MacCready will dislike any response other than "With Pleasure" (which he likes) when responding to Elder Maxson's order to kill Danse, and he likes it if Danse is killed, if one asks him for his thoughts afterward, he will be disgusted and say it was "low" of you to have killed him (no reputation penalty.)
 * Strong wants Danse to die just so he can see someone be killed.
 * X6-88 remains neutral in the situation as he himself hunts synths. He confesses that even he didn't know Danse was a synth. He encourages the Sole Survivor to let Danse live as killing him would be a waste of an asset.

Bugs
Blinder Verrat Слепое предательство Сліпа зрада