Contains 6 new levels

And a brand new lava costume

You can find a flashing light spot in the forest on the left of the first map after skipping the game help
Visit that light spot and you can enter the level of lava dungeon
Or
After the perfect ending,
you can visit the light spot next to the boat to return to the starting map again,
and then choose to visit the light spot in the forest on the left to enter the DLC map