Today I will help you with secret achievement in Cuphead (you must have DLC).
How to Defeat Secret Boss and Obtain Achievement
After you start doing things in next steps you must buy and equip Broken Relict.
In the next step you must go to this three guys.
Then talk to everyone, should start from first place guy to third place guy.
Every guy have other answer to other Cuphead’s players.
After talk with everyone you should remember answers about place. On my game save, first guy said: “… downright …”, second guy said: “… left …” and third guy said: “… down…”
After all of this things, you can go to graveyards field. With my answers I must go to this graveyards in first to third. (If you opposite the graveyard you should use on keyboard “Z” or on joystick “A”
If you corectly did this things you should see this blue light.
After beating boss you will unlock secret achievement and new charm -> Cursed Relic.
This charm changes your projectiles randomly with every shot!