(1) The trip back to the real Blackmarsh comes with two unexpected passengers: the baroness, who lurks somewhere near the manor gates, and Justice, whose spirit has somehow crossed the divide and joined with the body of the dead Grey Warden, Kristoff.
The new Justice decides to work with you against the baroness and enchants your weapons so that they can attack the Fade Portals that the baroness has ripped in the Veil. Be sure to loot the First's body for the powerful Armor of the Sentinel and the Mother's Chosen greatsword.
(2) The baroness's Fade Portals release more and more creatures from the Fade into the physical world. You must shut these portals down one by one. Head west from your return point and you'll see a Fade Portal almost immediately. A revenant and shade guard most portals, and the longer you wait on destroying the portal, the more creatures will pour forth.
When engaging, send your tank to occupy the revenant, ignore the shade, and put all three others on the Fade Portal to destroy it as quickly as possible. Once the portal is gone, cut down any creatures that have spilled forth and finish off the revenant as a group if it's still standing.
(3) Repeat your Fade Portal destruction on the second portal. Rest and recuperate before moving on to the next portal.
(4) Repeat your Fade Portal destruction on the third portal. Rest and recuperate before moving on to the next portal.
(5) Repeat your Fade Portal destruction on the fourth portal. Rest and recuperate before moving on to battle the baroness.
(6) Open the gate to the village area after all the portals have been destroyed. The baroness waits in the middle near the manor gate. Prepare for a battle royale. The baroness morphs into a huge pride demon, armed with several attacks meant to cripple your party.
The most dangerous is her ability to open up new Fade Portals. Shut these portals down immediately, even if means leaving yourself exposed to the baroness for a few seconds. If you let these Fade Portals fester, they'll begin pouring out creatures that will soon overwhelm you.
The baroness herself can blast away with fire bolts and frost bolts. If you have high resistances to fire and cold, you should be fine. If not, the party healer will have to watch for one of these attacks and immediately throw a Regeneration or Heal after it lands.
A shockwave attack from the baroness deals normal damage and knocks you off your feet if you fail a physical resistance check. Even worse, her drain ability consumes health from the target she grabs and transfers that health to the baroness.
Keep up the constant pressure and stay patient while you chip away at the baroness's health total. The fight will be long, and the healer will probably tap into a few lyrium potions to keep up with the healing. With the right teamwork, you will survive her barrage of attacks and finally bring the baroness down.
(7) Search the baroness's corpse for nice loot (Firestompers, Soulbound ring, and 14 sovereigns) and the Dock Storage Key. Use the key to open the gate that leads out to the docks at the northeast corner of the manor. You gain a lot more cool loot from the docks, and if you're completing side quests, the fifth dragon bone rests here, as does Ser Alvard's Sword in one of the decomposing crates.
You've now completed the "Shadows of the Blackmarsh" quest line and can have Justice join your party as a burly warrior tank! Now it's time to return to Vigil's Keep for your next major quest.