A Study in Crimson is Vicho's murder investigation in Svartrau, played at night across the rooftops. It turns Vicho into a witchcraft vendor and hides a secret ending tied to his own past.
Visit Vicho's Emporium after dark and go in through the balcony door. Find the source of the noise inside and you meet Vicho. He cannot go out in sunlight, so come with plenty of night left.
Follow Vicho over the roofs to a jeweller's house and inspect the two bodies. Use the jeweller's key on a small box under the stairs. Then search Doctor Simeon's house, take the property deed and climb into the warehouse it names. Simeon waits on the bottom floor.
Simeon has turned into a murohn, and you fight him with Vicho. The story carries on the next day in The Good Samaritan, and you later confront Vicho at night about what you learned.
Finding out the truth about Vicho earns the hidden achievement The Unquenchable Thirst. Guides tie the trophy to the follow-up, The Good Samaritan. If you tell him the truth, he chooses to end his life, and his house then holds the legendary amulet Vicho's Cross.
Expect about 750 XP across the objectives, a skill point and the Mercenary's Amulet. Your answers after the Simeon fight do not change the outcome.