The End Begins
Go and speak to Summer near the Wilderness Wall to start the quest. She'll immediately take you to the farm. Enter the portal inside the building then you'll be surrounded by familiar faces. They inform you that the Beast has grown stronger, and if nothing is done, it'll break through the Spirit Realm and into the real world.
You must lure the Beast to the cave entrance pictured above. This time it has to be with a Familiar that uses a Blue Charm to make its pouch. You might want to take multiple pouches in case it eats the one that you use.
Before you lure the Beast, however, you need to go and destroy the entrance to the cave in the real world. This will require a Pickaxe. Go to the real world and enter the cave. Once inside, loot the corpse. You'll obtain a Bronze Pickaxe, Mithril Hatchet, Spade, and Tinderbox. Once you have these, go back outside and use the Bronze Pickaxe to collapse the entrance.
Luring the Beast
The next step is to grab a few Blue Charm Familiar pouches. Next, go back to the cave entrance in the Spirit Realm. Make sure that your Familiar is closer to the path that the Beast takes, and position yourself closer to the cave entrance. You need to enter the cave as the Beast targets your Familiar. If you're too late, he'll devour it. If you do it right, he'll enter the cave with you.
Root of the MatterMake sure that you have your Hatchet and Tinderbox with you for this part. Having lots of food is suggested for this part as well.
Note: If you die while you're fighting the Beast, your gravestone will appear at Summer's location near the Wilderness Wall.
Once inside, you'll need to light pyres to distract the Beast so that it can be attacked. Cut the Cursed Willow Logs that are needed from the roots inside the cave. Fill the pits with logs as you can for the pyres. You might want to build all three pyres before lighting any as this is easier. Watch for your screen to shake, and listen for a special sound, these will tell you that the Beast is about to attack you. When you see the shaking, run away. The attack will hit the spot you were at and then shoot off two or so more blasts into the area. When this is finally finished, you'll witness a cutscene.
The next room is less difficult than the first one. You must go into the next cave area and be prepared to fight a different version of the Beast. You'll need to run around and avoid attacks until his core comes out, then dig in one of the Soft Soil spots and lure the core into the hole. You must do this three times in order to beat this challenge. When you finish this room, you'll see another cutscene.
A Grave Ending
This next room is the final one. Not only will it be the tomb for the Spirit Beast, but it currently houses the graves of your three friends. You must bless their graves at the right time to harm the Spirit Beast. Each grave will get a soul with a specific color. When the fight starts, observe which color goes into which grave. Bless the grave that has the color that the Spirit Beast is as he is getting ready to attack. If done right, the attack will harm the Spirit Beast instead of the heroes.
You may wish to pay attention to only one color at a time. Each color can only be effective once, so when you bless one color, ignore it for the rest of the fight. Once each of the three colors has done damage to the Spirit Beast, it will be defeated.
Make sure to keep your Prayer Points up during the fight by using the altar in the cave. It'll keep track of your countered hits on it, so if you keep dying, just make sure to not use a color that you countered him with before.
Once you have beaten the Beast, Summer will come and talk to you. She will tell you to go and speak to the Elders. They're in the cave with you and Summer. Follow them outside to get your reward.