The Underworld

The Underworld is a secret world appearing in Mr. Hopp's Playhouse 2 that can only be unlocked by collecting the Six, which are scattered throughout the Blacklands Manor Orphanage.

Background
The Underworld is where the Entity was sent after it was banished from the Above.

Mr. Hopp's Playhouse 2
Esther arrives in the Underworld after she returns from Ruby's house, falling through the floor after the Six cause it to break.

The Underworld has six stages that Esther must traverse before the boss fight against the Amalgamation. When traversing the Underworld, there are many Shadow Creatures that attempt to attack Esther. Fortunately, Esther gains an ability granted to her by the Six that is utilized to repel the Shadow Creatures.

Esther makes it to the end where she finds several matrons and children of the orphanage, including Molly and Isaac. Esther is happy to see them, but then has to battle with the Amalgamation before eventually defeating it along with the Entity and Isabelle. Upon the three monsters' defeat, everyone including Esther is freed from the Underworld and sent back to the orphanage.