The fifth world of New Super Mario Bros. 3 (Silver-zoroark's version). This World takes place in a spooky forest, and is the second last World. It has at least 2 levels with Boos in them.
Takes place outside at night, in the spooky forest. Splunkins and Boos are enemies here, also with Balloon Boos.
An under-water level with Unagi, the eel-like creatures. It is featured in a cave but almost the whole level is under the water. Frog Suit is featured.
Level 5-Ghost House
It is not suprising that a spooky World such as this would have a Ghost House, but this is probably the most difficult of them all. There are lots of doors as well as the floating Helping Hand, in a secret door which guides Mario to the secret exit.
This level can be accesed by taking the secret exit from the Ghost House, but it is mostly nightmare-ish. Like a level in New Super Mario Bros. Skeeters are dropping down from the sky, and they are literally the only enemy in the level. If the player would ever manage to beat this level, they would unlock the cannon to World Gold, which is quite a great achivement.
The Tower is filled with platforms that change places every five seconds. Enemies like Bone Goombas, Dry Bones and Bone Pirahna Plants are the only enemies in the tower besides Amps, who are on the rope-like "Cords".
This level is a level with ice, and features Splunkins but no Boos. Broozers everywhere make up for this.
An underground level with Buzzy Beetles and Bob-Ombs. There are bottomless pits practically everywhere, so it is a fairly hard level. And the level also introduces Kab-Ombs, who only are there for a short time.
A level in the sky with dark clouds. It is yet another self-scrolling level, with Lakitus and Bullet Bills. There is also a moving platform that goes up and the level has Fuzzies again.
This castle has the platforms from the Tower, and also features giant Cogs and more Cords. Amps are in this level a lot, and at one point there are Snake Blocks. Also the boss is King Boo.
Easy - 5 Small Splunkins and 2 Big Splunkins
Medium - 4 Boos
Hard - 1 Boohemoth