Fans of Undertale know that Toby Fox likes to hide a lot of secrets in his games, whether they be hidden items, secret enemies, or entire villages. One of the harder things to figure out in his new game, Deltarune, is a secret boss that is much harder than any other enemy in the game. For those who want to take on this challenge, follow this guide to figure out how to get to this secret boss.
Before we get started, however, keep in mind that this secret boss doesn’t become available until pretty far into Deltarune, so SPOILERS are inbound. It is recommended that you play Deltarune blind first, then start a new game in a different save file to try out other things, like trying to take on this boss. But, of course, that’s up to you.
Your journey to the secret boss will begin after you’ve escaped from prison. Once Susie busts you out, your party will automatically move down a screen from the cell you were kept in. From here, go right a screen. You will find a save point, to the north of which will be an elevator door. Enter the elevator. At this point, Ralsei will take the party to Floor 1F and dialogue will ensue on the trip there. Once you’ve reached your destination and the dialogue ends, Ralsei and Susie will leave the elevator. Instead of following them, stay in the elevator. Interact with the elevator’s buttons and select “??????” as your destination. The elevator will descend to your new destination. Once there, leave the elevator and head down the stairs to the right side of the screen. At the bottom of the stairs, past another save point, there will be jail bars. Interact with these bars to talk to the prisoner. He will offer for you to play a game with him. Choose to play the game. This will initiate the next step in the process, for which you will need to find three Broken Keys. Once you’ve finished talking to the prisoner, go back up the stairs, into the elevator, and return to Floor 1F. Once you’ve reached this floor, leave the elevator and continue downwards until you’ve moved down a screen. Continue going down until you appear on the right side of the screen, outside the castle. Here, you will find one of the fast travel doors that appear in Deltarune.
To retrieve Broken Key A, you will need to use this fast travel door to return to the Field area. Once here, go left a screen and enter the shop. Talk to the shopkeeper and select the “Strange Prisoner” option, which will be highlighted in yellow. The shopkeeper will briefly discuss the danger of the prisoner before deciding that some chaos might be entertaining, after which he’ll hand you Broken Key A. He’ll then give you a somewhat cryptic clue as to where to find the next Broken Key, but if you’re following this guide, you can ignore his clue. After receiving the Key, go right a screen to return to the fast travel door.
For Broken Key B, you’ll need to use the fast travel door to return to the Bake Sale area. From here, go right a screen, then go right another screen. You should now be in a room with dancing enemies that cause stars to appear around them when you get close. Move right, getting past the first set of dancers. At the second set of dancers, you’ll be able to either move down or continue right. Move down. Once past this set of dancers, you’ll have to move left, where you’ll find a third set of dancers. This set is made up of only four dancers. As you’re moving past this set of dancers, hug the bottom of the square they’re dancing around. If done correctly, you should find a hidden pathway you can walk down. Continue downwards until you’ve moved down a screen. Here, there will be a straight pathway heading right. Follow this pathway. There’s an enemy that will be in the way that is very hard to dodge, so you’ll probably just have to battle it. After that, continue right until the path takes you up a screen. Here, there will be a box that you can open to obtain the second Broken Key. Now backtrack until you get back to the fast travel door.
To get Broken Key C, use the fast travel door to return once again to the Field area. Once exiting the door, go right a screen. From here, go up through the gate and up a screen. Go up a screen again, then walk right until you find a card suite puzzle. The solution to this puzzle is found in the Castle area toward the end of Deltarune, but you can obviously just follow this guide for the answer instead of trying to figure it out yourself, since that’s what it’s here for. You’ll want the puzzle to read: Diamond, Heart, Club, Spade. To get this outcome, press the buttons in the following order:
Once the puzzle has been solved, the spikes will lower, granting you access to a box that is holding the third and final Broken Key. With all three Broken Keys in hand, return to a fast travel door.
We’re in the final stretch, now. Use the fast travel door to return to the Bake Sale area, then go up a screen. Here, you’ll find Malius, the smith who can fix anything. Talk to him and ask him to fix an item. He’ll notice your Broken Keys and fix them, creating a single Prison Key. Now use the fast travel door one last time to return to the Castle area. Go right a screen, then continually up until you enter the castle again. From here, stick to the right to get back to the elevator. Use the elevator to return to the “??????” floor. Go back downstairs and talk to the jail bars. When given the option to use the Prison Key, use it. A door on the bars will open, granting you access to the secret boss. There’s a save point right there, which you’ll probably want to use, because this boss is by far the hardest enemy in Deltarune. Once you’ve stocked up on all the weapons, armor, and healing items you want, enter this door to the right to initiate the battle with Jevil, the secret boss.