She will be in an unmarked location in the middle of a path. Catch up with her briefly and escort her to the mountain's edge. Deimos will be at the edge. This is the last chance to kill or save Deimos.

How do I get back my Deimos?

In A Bloody Feast, tell Myrinne “I'll bring Deimos back." In A Bloody Feast, tell Deimos "Hold on!" after you fight him. In Doing Time, convince Deimos the cult is manipulating you both. Deimos will say "I am not your puppet," if it worked.

How do you convince Deimos in Chapter 8?

You must now choose the following options to win Deimos over: “It was the prophecy” and “You and I both know who feeds the Pythia her words”. Deimos will acknowledge your words and is ready to listen for more. Next, you must choose 'Our mother searched for you', 'She thought you were dead'.