World 8 (New Super Mario Bros.)
From the Super Mario Wiki, the Mario encyclopedia
World 8 is the final world of New Super Mario Bros. Mario and Luigi travel to this world after defeating Monty Tank or Lakithunder, or using the cannon in World 5. This world is divided into two parts: a "dark forest" area, and a "lava" area; both having their separate towers and castles. The two bosses of this world are Dry Bowser; and a resurrected Bowser fighting along with Bowser Jr.
The first half of the world has a "spooky" theme; featuring Splunkins, Scuttlebugs and Crowbers, and featuring the same music as the Ghost House levels. Notably, this is the only world to have Boos in a non-Ghost House level. The second half introduces rising lava and unique enemies such as Sledge Bros., Fire Bros., Kab-ombs, and Volcanic debris.
Unlike all the other worlds, this world does not feature secret exits or unlockable courses. One of each type of Toad House can be found in the forest area, but none can be found in the lava area.