Major spoilers for ending of BG3:
!This is my first playthrough where Lae'zel wants to free Orpheus (and not just to immediately kill him) so the choice of whether or not to do so is harder than just "do I trust the Emperor or not?". I'm definitely not the first to feel weird about the options you're given here and how they play out in terms of The Emperor's actions, but I thought it'd be fun to write out my headcanon for the direction I wish I could have taken the conversation (a direction I believe would stay true to what Larian intended with this scene). !<
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!After your character indicates they want to free Orpheus and The Emperor gives its speech about trust, your character can say "I trust you, but we can't let Orpheus be killed. We need to find another way."!<
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!The Emperor responds "There is no other way. An Illithid is needed to dominate the Netherbrain, and I cannot exist outside of the astral prism without absorbing the protection provided by Orpheus."!<
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!You can respond "What if Orpheus is willing to protect you? He could be freed and we could still be shielded from the Netherbrain."!<
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!The Emperor: "I told you: to Orpheus, you are Illithid. If you free him, he will kill you. Right after he kills me."!<
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!You can then make a persuasion check with a DC influenced by how much you've shown The Emperor trust thus far: "He will see reason. And if he doesn't, I will subdue him again. I won't let him hurt us."!<
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!If unsuccessful, The Emperor doesn't believe you, and you're brought back to the original dialogue options.!<
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!If successful, The Emperor will guardedly agree to let you free Orpheus. However, it doesn't trust that Orpheus will protect it if it leaves the astral prism, so it will refuse to go with you.!<
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!When Orpheus is freed, he will initially be hostile toward The Emperor and threaten to kill it for imprisoning him. You can pass a high persuation or intimidation check to convince him to see reason and work together for the common cause. Otherwise, you can side with him against The Emperor and help him kill it, or you can fight Orpheus. If the latter, you must knock him out; if you kill him, you get a Game Over.!<
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!If you convince Orpheus to stand down, you are then presented with the same choice as you would if you'd freed Orpheus without convincing The Emperor to stay. However, The Emperor becomes a powerful callable ally in the ensuing battle. It continues communicating with your party, and it has a tense but nonviolent interaction with Orpheus after the Netherbrain is defeated.!<
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!If you subdue Orpheus, Lae'zel is angry, but she will reluctantly agree in light of Orpheus's hostility that The Emperor consuming him is the only option.!<
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!If you help Orpheus kill The Emperor, the scene will proceed as if The Emperor abandoned you.!<
!What do y'all think? Does this seem realistic, or am I off with any of the characters? What's your headcanon for this scene? !<
So, since it's you're first play through, you might be missing a bit of information/lore.
spoiler
The Emperor, if you select certain dialog options in one of the 1:1 conversations with him, he's completely self serving and has been lying to you from the beginning. He sees you as his thrall, as a means to an end. A lot of the backstory he's told you, as many of the interactions with him, has been half truths at best. There's no way he'd partner up with Orpheus as Orpheus is going to see right through the lies and want him dead. Orpheus wants you dead too, but due to the threat at hand reluctantly partners with you. And the Raphael is the one that imprisoned Orpheus in the first place, so once released, the Emperor is screwed.