Memory of Scholomance

Enter Heroic Scholomance and defeat Rattlegore. Doctor Theolen Krastinov has a chance to spawn after Rattlegore is killed. Defeat him to receive Krastinov's Bag of Horrors, a toy required to begin this process.

Enter Scholomance and defeat Darkmaster Gandling. After the fight, go upstairs to the southern room and approach the bone pile with a bright bone sticking out of it to spawn Eva Sarkhoff. Right-click off the toy's buff before speaking to her, as the disguise will prevent interaction. She will give you Inert Spectral Essence and task you with performing a ritual from her journal.

Travel through the Plaguelands to gather the five ritual items needed for the candle trade: Caer Darrow Fountain Water, found as a bucket on the fountain in Caer Darrow at 68.8, 78.9; The Deed to Andorhal, on the wall in the main room at 43.62, 69.28; Trampled Doll, right next to the wheel of a cart at 35.71, 83.68 in the next zone; The Road Ahead, on the wall in the tavern at the end of the bar desk at 54.99, 64.1; and Undelivered Shipment of Smokes.

Trade the five collected items with the dead NPCs near Scholomance to receive the five candles needed for the ritual: Stinky Candle, Authentic Andorhal Candle, Ghost-Warding Candle, Imported Candle, and Tobacco-Filled Candle.

Enter the building entrance to Scholomance and turn right instead of left toward the dungeon. Go all the way back to the room with bones on the ground and a grate in the middle of the floor. Use Eva's Journal there to complete the ritual, which will trigger the Memory of Scholomance achievement and permanently unlock the original vanilla Scholomance instance.