Nevill said:
Fangshi, what is Derryth's attitute towards TFM she'll be going with in D?
To kill him primarily. Failing that to weaken him enough that she can force him back into the bracelets.
Willingly freeing the creature is in essence a declaration of war and she is fairly certain she knows how he will respond.
She does not really believe she can talk sense into him though. She will try to threaten him, she will invoke Mazzarin's name and try to force him to comply. It might work, depends on how angry he gets really. She is certain that she will not be able to get him to agree with her willingly, he is set on this course of action and the only way to stop him is by removing the possibility.
She knows this because she understands why he is acting this way. He has been trapped for years, maybe decades, in the bracelets and this is the first real chance he has had at freedom. She is not certain that she would not do the exact same thing...
She is also quite confident that if she tries to stop him and fails he will attack them and carry out his threat. Not necessarily because he will hate her but because he said he would do it and at that point it would be the principle of the thing. She is fairly certain that he would view it as a 'lesson' for them and little more.
Best outcome for Derryth in D is that he dies and solves her problem.
Second best outcome is that he loses and/or is so weakened that she can force him to return to his prison. (Only really possible in Di)
The worst (and arguably most likely outcome) is that he wins and does everything he said he would...
Any negotiations that do take place will be tense ones. The working relationship Derryth has with the Faceless Man will not survive any attempts by her to sabotage his efforts but that goes without saying.