Time Passed Since Clearing "In the Shadow of the Relic"
Map Location
Exterior
"In the Shadow of Fate" is triggered by the passage of time after clearing "In the Shadow of the Relic" as a Post-Game Quest. You need to progress through the main quest until near the end, so if it doesn't trigger, finish the story. "In the Shadow of Fate" can be accepted in the underground cathedral.
The ending of "In the Shadow of Fate" branches depending on the choices made. If Sebastian is handed over, he will be expelled. If he is not handed over, he will remain enrolled. It is also an option for those who want to see how the story unfolds.
Expulsion results in inability to learn Unforgivable Spells
If Sebastian is handed over and expelled based on the choices made in "In the Shadow of Fate," he will no longer be able to learn the Unforgivable Curses. If you have not learned the three spells "Crucio," "Imperio," and "Avada Kedavra" in a previous quest, be sure to choose "Cannot hand over Sebastian."
If Sebastian is not handed over in "In the Shadow of Fate," you can learn the Unforgivable Curses by talking to him in the Underground Cathedral. Those who have missed learning the Unforgivable Curses should definitely have a conversation with Sebastian.