Deltarune: All Secret Bosses and How to Unlock Them

Every chapter of Deltarune features an optional secret boss that rewards players willing to explore beyond the main story. These encounters are among the toughest fights in the game, often requiring hidden quests, puzzles, or obscure collectibles before they become available.

In addition to unique weapons and armor, defeating each secret boss awards a Shadow Crystal—a mysterious item closely tied to Deltarune’s overarching story and one of the game’s most sought-after collectibles.

Jevil (Chapter 1)

Jevil is the first secret boss players can encounter. Once the court jester of the Card Kingdom, he was imprisoned after meeting a mysterious figure who convinced him that the world was nothing more than a game. Believing himself to be the only truly free person, Jevil embraces complete chaos, making his battle one of the most unpredictable in Chapter 1.

How to Unlock Jevil

To challenge Jevil, you’ll first need to forge the Prison Key.

After reaching Card Castle, take the hidden “??????” floor using the southeast elevator and speak with Jevil inside his prison cell. You must then collect three key fragments:

• Receive one fragment from Seam.
• Find another along the invisible path near the Paper Dancers.
• Solve the card suit puzzle using the sequence Diamond, Heart, Club, Spade to obtain the final fragment.

Bring all three pieces to Smith Malius, who will forge the Prison Key and allow you to unlock Jevil’s cell.

Rewards

Jevil’s fight is known for its relentless bullet patterns and fast-paced attacks.

Depending on how you complete the encounter, you’ll receive:

Both outcomes also award the Chapter 1 Shadow Crystal.

Spamton NEO (Chapter 2)

Spamton quickly became one of Deltarune’s most iconic characters. Once an unsuccessful salesman living in Cyber World’s Trash Zone, he dreams of becoming a “BIG SHOT” and escaping the unseen forces controlling his fate. His transformation into Spamton NEO leads to one of the game’s most memorable boss fights.

How to Unlock Spamton NEO

Unlocking the encounter requires completing Spamton’s hidden questline.

First, accept Spamton’s final deal during your initial meeting instead of defeating him immediately.

Next:

1. Wait until the KeyGen drops below 100 Kromer in Spamton’s shop.
2. Purchase the KeyGen.
3. Use it to access the basement beneath Queen’s Mansion.
4. Find the EmptyDisk inside the abandoned machine.
5. Return it to Spamton to receive the LoadedDisk.
6. Insert the LoadedDisk back into the basement machine.

Leaving the room triggers the Spamton NEO battle.

Rewards

Players receive different equipment depending on the route taken:

• Dealmaker
• Puppet Scarf

The fight also rewards the Chapter 2 Shadow Crystal.

The Roaring Knight (Chapter 3)

Unlike previous secret bosses, Chapter 3’s encounter isn’t hidden behind a lengthy quest. Instead, the mysterious Roaring Knight interrupts the chapter’s finale after effortlessly defeating the main boss before turning on the party.

The real challenge lies in preparing for the battle.

How to Prepare

The recommended strategy is to obtain the Shadow Mantle, a defensive item that greatly improves your chances of surviving.

To unlock it:

• Earn S Rank on both TV Time boards.
• Visit the S Rank changing room several times.
• Speak to the NPC in the upper-left corner during three separate visits.
• Complete the hidden mini-game and defeat the secret shadow battle.

Successfully completing the challenge rewards the Shadow Mantle.

Rewards

Defeating the Roaring Knight grants the Chapter 3 Shadow Crystal.

Hammer of Justice (Chapter 4)

Chapter 4’s optional boss is the Hammer of Justice, the Darkner form of Gerson Boom. Clad in heavy armor and wielding a massive war hammer, this encounter requires players to complete several hidden objectives before it becomes available.

How to Unlock the Fight

Start by defeating the Jackenstein mini-boss to obtain the ClaimbClaws, which allow you to climb special book ladders.

Next:

• Collect three hidden music pieces found throughout optional areas.
• Return to the golden piano above the Study.
• Play the correct melody to open a hidden passage behind the fireplace.

The secret arena lies beyond the hidden door.

Keep in mind that healing items cannot be used during this battle, making preparation especially important.

Rewards

Defeating the Hammer of Justice rewards one of Susie’s strongest equipment upgrades along with the Chapter 4 Shadow Crystal.

Mad Mew Mew (Pink) (Chapter 5)

The Chapter 5 secret boss is Mad Mew Mew (Pink), a Darkner version of the Mew Mew Kissy Cutie doll. Hidden behind a locked pink door near the end of the chapter, she continues the series’ tradition of difficult optional encounters paired with cryptic lore.

How to Unlock Mad Mew Mew

Before the fight becomes available, you’ll need to collect all 10 Pink Coins scattered throughout Chapter 5.

Some coins are hidden behind optional puzzles, while others require abilities obtained later in the chapter, making backtracking necessary.

After collecting every coin:

1. Purchase the Mysterious Key from the Cliff Shop.
2. Instead of heading directly toward the Dark Fountain, travel left from the top of the castle.
3. Drop into the hidden pit.
4. Follow the concealed path to the locked pink door.

Using the key unlocks the battle.

Rewards

Mad Mew Mew features some of the most complex bullet patterns in Deltarune, making her one of the hardest optional bosses released so far.

Defeating her awards the Chapter 5 Shadow Crystal.

Every Shadow Crystal Reward

Why Shadow Crystals Matter

Each major secret boss rewards a Shadow Crystal, making them one of the most important collectibles in Deltarune. Although creator Toby Fox has not fully explained their purpose, the crystals appear closely connected to the mysteries of the Dark Worlds and the Roaring Knight.

Whether they’re ultimately required for hidden story content remains unknown, but collecting every Shadow Crystal is widely considered essential for players aiming to uncover all of Deltarune’s secrets as future chapters are released.