Finding the secret boss in Toby Fox's latest release isn't just about bragging rights; it's about not missing some of the best writing in the series. Honestly, if you've played Undertale or the earlier chapters of Deltarune, you know the drill. There is always a hidden, terrifyingly difficult encounter tucked away behind a series of cryptic errands.
Chapter 4 is no different. The Deltarune Chapter 4 secret boss guide below will walk you through the steps without ruining the story beats or the boss's identity.
The boss in question is often referred to by the community as the Hammer of Justice. If you're looking for that specific Shadow Crystal or just want to see Susie get some of the best gear in the game, you've gotta put in the legwork.
Don't Miss the Window: The Point of No Return
Before you dive into the puzzles, there’s a massive warning you need to heed. Do not progress the story past the point where the party prepares to leave the Dark World.
Unlike Chapter 2, where you had a bit more flexibility, Chapter 4 has a narrative "lock" that prevents you from backtracking to certain areas once the final sequence begins. Most players suggest finishing the main dungeon's puzzles but stopping right before the climactic confrontation with the chapter's primary antagonist.
If you miss this window, you’re looking at a full replay of the chapter. Don't be that person.
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Step 1: Getting the ClimbClaws
You can't even reach the secret area without a specific traversal item. To get it, you need to defeat a mini-boss named Jackenstein.
This isn't an optional fight; it's part of the main progression. However, what is optional is talking to him afterward. Once the battle ends and the dialogue plays out, make sure you interact with him to receive the ClimbClaws. These allow you to scale the "book-ladders" found throughout the Dark Sanctuary.
Step 2: The Search for the Three Music Pieces
This is the "meat" of the quest. You need to find three separate sets of musical notes to play on the Golden Piano.
The First Piece: The Watery Path
Head to the Study and take the lower western path. You’re looking for a room with patrolling "puffball" enemies. Use your new claws to scale the wall in this area. At the top, you'll encounter a minor mini-boss. Defeat them, and you’ll find the first set of notes.
The Second Piece: The Stained Glass Hallway
Go back to the central hub area near the bookcases and the stool. This time, head east through the Stained Glass Hallway. Look for another climbable wall. Once you’re in the pitch-black section, look for a faint, glowing figure in the lower-right area. Follow the path they open and hit the yellow switch. This reveals the second piece.
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The Third Piece: Behind the Waterfall
This one is the trickiest. Go to the landing just below the Golden Piano. Take the western door into the library area. Look for the waterfall—there’s a secret passage behind it.
Pro Tip: Some players report that you need to donate roughly 100 Dark Dollars to the nearby fountain to trigger the passage opening, though this is still being debated by data-miners.
Once inside, you’ll find a maze of bells. Navigate to the end of this section to grab the final set of notes.
Step 3: The Golden Piano Puzzle
Now that you have all three pieces, head to the top of the map where the Golden Piano sits.
You don't actually have to memorize the notes. If you’ve collected all three items, interacting with the piano will automatically trigger the "correct" performance. A cutscene will play, showing a secret passage opening elsewhere.
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How to Start the Fight
With the piano puzzle solved, go back to the Old Man's Study. You’ll notice the fireplace has shifted, revealing a dark tunnel.
Save your game here. Seriously. This fight is a massive jump in difficulty compared to the rest of Chapter 4. One unique mechanic to keep in mind: you cannot use standard inventory items during this encounter. The boss forces you to rely entirely on ACTs and specialized battle mechanics.
Battle Tips for the Hammer of Justice
- Soul Mode: Be prepared for the Green Soul mechanic. If you played Undertale, this will feel familiar, but Toby adds a few "Deltarune-style" twists that involve moving the shield in 360 degrees.
- Susie is Key: Much like the Chapter 3 boss, Susie is your primary tank and damage dealer. Ensure she has the best armor available before entering the fireplace.
- TP Management: Since you can't use items, your TP is your lifeblood. Spend your first few turns Grazing as much as possible to build up a surplus for healing ACTs.
Rewards and Next Steps
Beating the boss (either by Fighting or Sparing) nets you a Shadow Crystal and the Justice Axe. The Axe is a massive upgrade for Susie, significantly boosting her Attack and Magic stats.
Once you've finished the fight, don't forget to visit Seam in the Hub world. He’ll have new dialogue regarding the crystal and might drop a few hints about what's coming in Chapter 5.
If you’ve followed this Deltarune Chapter 4 secret boss guide correctly, you should now have one of the rarest items in the game. From here, your best move is to head back to the main path and finish the chapter to see how your victory affects the final cutscenes.