Acquisition
This will become available after completing the Night Terrors quest if Merrill accompanied you into the Fade and accepted the Pride Demon's offer to betray Hawke.
Walkthrough
Go to Merrill's house and listen to apology.
Merrill: friendship (+15) for choosing the kind responses.
Merrill: rivalry (+5) for choosing a sarcastic responses.
Merrill: rivalry (+10) for choosing a confrontational dialog options.