After defeating the Stygian (The Ashworm ) , return to the Ashland again and go to this part of the map :






Start Jumping into it and climb outta it with the demonic growth on the it's walls .
Everytime you do it 4 horsemen will spawn , Do it for 25 times and you will have 100 of em .
well after that approach them and trigger all of em so they gonna chase you down , lure the horsemen to this cave here :



Then hop onto the rocky platform and start killing all of the horsemen