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The Hammer of Justice is the secret bonus boss of Chapter 4 in Deltarune — and he’s a very different kind of boss. To encounter him in Chapter 4, you’ll need to find three hidden piano pieces to play on the Gold Piano accessed from the Dark Sanctuary Study where the Old Man is found. Defeating the boss gets you a Shadow Crystal and the Justice Axe which is Susie’s strongest weapon so far. This is a tough fight you can only take on with Susie by herself, so you’ll need to practice hard if you want to win.
Here’s how to unlock the big optional boss in Chapter 4. If you’re still working through earlier chapters, here’s how to earn the Shadow Crystal in Chapter 3, or how to fight NEO Spamton in Chapter 2.
Hammer of Justice | Secret Boss Guide
The Hammer of Justice boss is found through a secret door in the Study — the central hub of the Dark Sanctuary in Chapter 4. To unlock the path to the secret room you’ll need to fully explore the first Dark Sanctuary after acquiring the climbing gear. After defeating the Jackenstein boss, you’ll be able to fully explore the Dark Sanctuary area — don’t progress too far or you’ll lose the chance to find all three music pieces.
- Secret Boss | Chapter 4 Steps
- Defeat Jackenstein in the Dark Sanctuary World and acquire the climbing gear.
- With the climbing gear, there are three optional areas you can now access. Each one has one-of-three parts of a music sheet.
- Go to the gold piano above the Study. It’s through the upper door — then climb the books to the right of the save point.
The three music pieces you need are located at the top of optional climbing challenges. Reach the top and the music piece will appear — these pieces DO NOT need to be memorized, and they will appear in the gold piano room.
- First Music Piece: From the Study, go left and follow the path down and left through the magic projectiles. Keep going until you find a climbable wall that leads up — this takes you to a long climbing sequence that ends with a miniboss. Win the fight to earn the music piece.
- Second Music Piece: From the previous music piece, return to the main path (with the magic projectiles you have to dodge) and go right. This leads to a long, stained glass window hallway. After the hallway, there’s a climbable wall that leads to a dark room full of statues. Navigate using sound to find a light switch that makes the second music piece appear.
- Third Music Piece: After climbing out of the Jackenstein boss area, return to the save point and go back to the piano puzzle room. In the hallway full of books, there’s a door to the left that leads to a fountain — go behind the fountain to find a dark path. Climb the wall of bells at the end of the path to reach a second dark passage. That’s where you’ll find the last music piece.
With all three pieces, go to the gold piano and play the notes that appear. They’ll appear in order once you’ve discovered the three parts. This unlocks the hidden fireplace door in the Study. Going inside and trying to take the Justice Axe will lead to the boss fight.
How To Beat the Hammer of Justice
The battle against the Old Man (the Hammer of Justice) is fought with Susie alone — equip gear to increase her magic. The Absorb Axe is useful here and boost her defense to maximum with any armor. You CAN NOT damage the boss. You just have to survive.
- The Hammer of Justice will dodge all of Susie’s attacks. DON’T BOTHER ATTACKING. Save all your TP for OkHeal — your healing is severely weakened, but it’s still extremely useful.
- DEFEND every turn to boost your TP. Use OkHeal whenever you can to keep Susie’s health higher. Using attacks or RudeBuster is useless here — neither will land and they also will not progress the battle.
- To progress the battle, you must take actions — each action will cause the battle to end slightly faster. Attacks are the weakest, Rude Buster is strong, and OkHeal gives the most progress but costs the most TP.
- Learning the Hammer of Justice’s patters is the most important skill. Once you can consistently avoid (most) damage, you’ll be able to survive long enough — use healing whenever you can, attack between heals (if you have the Absorb Axe) and Defend if you’re struggling to dodge.
This is a long boss fight that can seem incredibly unfair if you don’t know what to do. Just remember that healing counts towards progressing the boss — if you only defend and don’t act, the boss will get increasingly difficult until you fail. Your goal is to outlast him and “impress” him.
If you make it to the end of the fight, you’ll earn the Justice Hammer and one Shadow Crystal.
