The Dark Kind came to the world as conquerors. They failed. Now they are odd neighbors.
HeartKind bands are a way for Dark Kind (or any Deamonic/Monsterous kind) to show that people have nothing to fear from them.
As part of The Pact that ended the Six Stars Demon Wars, various places were given over to the Dark Kind. This is their neighborhood in the city.
All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.