The Chapter 4 secret boss in Deltarune is commonly identified as the Hammer of Justice , associated with Old Man Gerson in fan and guide coverage of the chapter’s hidden fight. Guides describe unlocking it by defeating Jackenstein, obtaining the ClaimbClaws, collecting three music pieces for the golden piano, and then returning to the Study to trigger the fight.

How it’s unlocked

  • Defeat Jackenstein and get the ClaimbClaws.
  • Use them to find three segments of piano music in Chapter 4 areas, including near the waterfall room and other puzzle spots.
  • Play the music on the golden piano above the Study.
  • Return to the Study and interact with the hidden fireplace path to start the boss battle.

What to expect

The fight is described as a secret boss encounter with special restrictions, including no healing items during the battle. Coverage from June 2025 and June 2026 presents this as the Chapter 4 hidden boss route rather than a standard story fight.

TL;DR

Chapter 4’s secret boss is the Hammer of Justice, and you unlock it by clearing a puzzle chain involving the ClaimbClaws, the golden piano, and the Study fireplace route.