Revelations

Revelations is a companion quest for Blackwall in Dragon Age: Inquisition.

""Blackwall has left the Inquisition. It appears to have something to do with the impending execution of a man called Mornay, one of the soldiers responsible for the Callier massacre.

Acquisition
This quest becomes available in Skyhold when the Inquisitor approaches the stables, only to meet an Inquisition spy who informs him/her that Blackwall has indeed disappeared and hands over a note missing from the reports, which might shed some light into the disappearance.

Walkthrough

 * Travel to Val Royeaux and follow the quest marker to attend Mornay's execution. A cutscene occurs where Blackwall interrupts the execution, and informs the crowd that he was responsible for the murdering of Callier family. His real name is Thom Rainier, a former captain in Celene's army. He was recruited to the Wardens by the real Blackwall, but when Blackwall died in a darkspawn ambush, Rainier assumed his identity.
 * Speak to "Rainier" in his cell in order to learn more about his past.
 * Discuss the situation with Cullen, who is waiting nearby the prison cells. Cullen hands the Inquisitor a report of Rainier, saying that the Inquisition could use resources to get Rainier into their custody. The Inquisitor can then choose to leave Rainier in the prison or get him out.
 * Optional: Order the advisors to find a way to release Rainier from prison. If the Inquisitor wants to judge Rainier in Skyhold, he/she must complete a short War Table mission called Rainier's Release: An Alternative.
 * Judge Rainier at Skyhold. The Inquisitor can either free him to make amends for his crimes, keep him in servitude or give him to the Wardens.

Results
If you free him and let him atone, Blackwall approves of that choice. If you keep him in servitude and "continue the lie", he greatly disapproves, which also shows in later dialogue with him.