It’s a very early unlock
A new DLC, a new Cuphead DLC island! While The Delicious Last Course adds a playable third character (Chalice) into the mix on top of additional equipment, the main draw is an extra DLC island with more bosses to fight. The good news: it’s really easy to unlock.
You just need to finish the first Mausoleum level
Since a lot of folks are likely starting up a new fresh save file for the DLC (or are buying the game for the first time), I figured it was worth pointing out how to get to the new island.
In short: to access the DLC island in Cuphead, all you need to do is finish the first Mausoleum level. The shortest path involves beating the initial “Run & Gun” (platforming) stage, Forest Follies. It’s denoted by a small diamond icon on the ground. After completion (you don’t need to get all the coins), a bridge will pop up to the east.
Head over it, then south, to get to the Mausoleum. This one is super easy to complete: you just need to parry (press the jump button while in the air with Cuphead or Mugman) the spirits before they reach the middle of the arena. They move slowly and there aren’t that many to defeat.
After that, a ferry NPC appears that brings you to the new Cuphead DLC island
After completion the Mausoleum will crumble, denoting that it was conquered. An NPC will appear holding an oar, and their boat will take you to “Island IV,” which is the location of the DLC.
If you load into an existing save file, the NPC should be available to take you to the DLC without an extra step.