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Heart World (赤い世界, Akai Sekai, Red World) is one of the main worlds accessible from the Nexus.
The world mainly has red footprints that make a noise when stepped on and red statues. The background is plain black and the backing track is simply the sound of a heart beating. When you use the Lantern effect in this world, it changes the background design.
Stepping between the two statues facing each other a few times will spawn a pink skull creature that moves very quickly away from you; equipping the Lantern effect will cause it to stop moving, and flourishing the Lantern repeatedly will drag it back to you. If you interact with it, it will take you to the Panel Passage (パネル通路, Paneru Tsūro); a tile-filled sub-area. If you interact with the heart hidden there, it will take you to a large beating heart that you cannot escape from. Using the Chainsaw on it will stop it from beating.
Stepping on the pair of shoes at the end of one of the footprint paths near the entrance will take you to The Deciding Street, and interacting with the book with a beating heart on it will take you to Valentine Land.
The Twintails effect used to be found in plain sight in this area, which you could get from the Twintail Monster, but now it is found in the Bazaar in the Deciding Street.
Nexus → Heart World