Confront the culprit behind the Vael family's murder.
You must complete Sebastian's first companion quest to gain access to his second companion quest, "Repentance". It will be available from the start of Act II. Handing this quest in will net you this achievement.
Note: Sebastian's gift for "Memento" can be found during this quest in the Harimann Estate and a glitch to gain 100% rivalry quickly for "Loyalty of the Prince" can be done at the end of the quest. See the specific achievements for details.