Guinea Pig

If Dr. Swansea survived your meeting, he will have new dialogue in Pembroke Hospital.

Go behind the Ascalon Club and down to the sewer entrance.

Follow the linear path to Fergal's boss chamber and leave through the other door.

Follow the tunnel and you will reach the Sewer Skals Territory. Use your blood sense to find a blood trail.

Follow the blood trail to Harriet Jones's door, interact with it, and then turn around.

Follow the blood trail back the way you came, emerging in the Finsbury Park Reservoir.

Continue forward for an encounter with the Mysterious Entity. You can speak with him to learn his name, Myrddin Wyllt.

After talking, dash up to the bridge. Following the blood trail from here leads to the final boss and a point of no return. Back up your save now if desired.

Continue forward for a scene with a mutated Harriet Jones.

Defeat Harriet Jones. Avoid her arm and watch out for poison damage. Keep your distance when she is surrounded by a gas cloud. Use your Mace to stun lock her. Get far away when she dies, as she explodes in a massive poison cloud.

The room will fill with blood, and the Red Queen's Avatar will emerge.

Equip your shotgun and wait until you can lock on before firing.

Evade the shadows she summons. When she rises into the air, switch to your pistol and keep your distance.

If your health gets low, use a serum.

If bullets don't kill her, use Blood Barrier to protect yourself, Coagulation to hold her in place, and the True Dragonbane to finish her off.

Defeating her unlocks the Unnatural Disaster trophy.