A heartless is a m-... creature born from darkness. They come in a lot of types, but the most common ones, the ones that tend to appear first on any world, are little black things. They look like hunched-over people with heart-shaped holes in their chest and long antennas.

[He'd ask for a pencil to sketch one, but he doubts papyrus has pockets...]

They hunt people and spread like wildfire until they destroy the world's heart, then they go to another world, and another...