Baldur's Gate 3: SECRET Option - Viconia DeVir Encounter (Shadowheart Spoiler)
A workaround if you prefer to not deal with the combat. The conversation with Viconia is optional and only if you do not pursue the Shadowheart quest line. Entering the back room triggers aggression if seen. If you choose to skip the conversation altogether then lockpick the back door or pick pocket Viconia (I didn't try this, but it should work) and have Shadowheart and your companions become invisible and walk through Viconia's chamber into the back room.