This page contains all the information you need on how to begin, and keep, a romance with Morinth.
Recruit Samara and complete Loyalty Mission
Morinth can be romanced by either Male or Female Shepard and will not affect any of your other ongoing romances. However, this romance doesn’t unlock the Paramour II Trophy. Before you can even begin romancing Morinth, you’ll need to recruit Samara to your squad by completing the mission, Dossier: The Justicar.
Once you’ve got her in your team, complete a mission, then speak with Samara to acquire her loyalty mission, Samara: The Ardat-yakshi. During the part where you’re getting up close and personal in Morinth’s apartment, say ““Surprise” followed by “Kill Samara” when your choice comes up.