You’ll find Goathenge in the giant field area. Bop one of the stones and watch them collapse like a set of hulking great dominoes to fulfil this quest.
Ascend the small tower in the field where your goat spawns and enter the doorway at the top to complete this quest.
Find the pentagram by Coffee Stain Studios and either summon 5 goat peasants with your goat queen powers (acquired from Goat Castle) or drag 5 unsuspecting humans onto the pentagram under the guise of hosting a barbecue to complete this quest and turn into demon goat.
Bleat 3 times. That's it.
Collect all of the golden goat statues. It just so happens that we have a Goat Simulator Golden Goat Statues guide covering this – how wonderfully convenient!