Give Sebastian a family heirloom.
Sebastian's gift can be found during his "Repentance" companion quest. You will travel to Harimann Estate. Progress through the mansion until the cut-scene. When the cut-scene ends, a waypoint marker will appear on your map. The gift can be found on a corpse in the wine cellar in the room with the exit into the dungeon. Speaking to Sebastian and doing the gift dialog will unlock this achievement.