This is an unmarked secret quest that is available through the Darkening of Tristam event.
Starting the Quest[edit]
Enter the Anniversary Dungeon, the Labyrinth, and make your way down to Level 9. Search the area to find the Black Mushroom. These can appear anywhere on the dungeon floor, so search it thoroughly. Interact with it to get the Rotten Mushroom. With it in hand, exit the anniversary dungeon and head back to Tristram.
Head out of town to the Northeast. Take the First turn to the Southeast. This will lead you to Adria's Hut.
LEAVE THE COW CORPSES TO THE RIGHT OF THE HUT ALONE. ELSE YOU WILL NEED TO START A NEW GAME TO COMPLETE THIS ACHIEVEMENT.
Inside it is the Cauldron. Head over to it and interact with it.
This will cause it to drop Witch's Brew. Head back to Tristam
Once back in town, it is time to revive a few people. Head to the Southwestern corner of town. You will want to find Farnham's Corpse. Interact with it. As long as you have the Witch's Brew then Farnham's Ghost will appear and drop Drunkard's Debt.
Head to the center of town and go to the North of the Fountain. You will find Ogden's Corpse. Interact with it to cause Ogden's Ghost to appear and drop Garda's Letter.
Head a short distance to the Southwest to reach Pepin's Corpse. Interact with it to summon Pepin's Ghost which will drop Healer's Prescription.
Next, head to the Eastern side of the fountain. There you will find Griswold's Corpse. With the prescription in hand, Griswold's Ghost will appear and drop Plan: Wirt's Leg.
Head to New Tristram and talk with Haedrig Eamon.
Use the Plans to teach them to him. After that, go into forge Weapons and choose find Maces. There you will find Wirt's Leg. To forge it you will need 10 Reusable Parts and 1 million gold.
Do not leave the Blacksmith. Go to the Salvage tab next. Select the salvage option and then salvage Wirt's Leg. This will reveal the last item: Map of the Stars.
Return to the Old Ruins and enter the portal to Tristram. Once there, return to Adria's Hut Tristram. To the right of it you will see 3 Dead Cow. Interact with them in this order: Middle (2), Left (1), Right (3).
Done properly, a new area will open to the right of the hut: Abandoned Farmstead. Head into it to reach the final part of this quest.
Head to the central Northern part of the farm. You will find Wirt's Stash there. Open it to get the item Royal Calf. Interact with the open to receive your reward: The Royal Calf Pet.