The Heartless, the primary antagonists of Kingdom Hearts, are a strangely terrifying enemy for an otherwise child friendly enemy, due in part to the way that they can't be fought by normal means and steal people's hearts. However, the original Shadows, who have appeared in every game in the series all the way to the upcoming Kingdom Hearts: Melody of Memory, open up for one of the most important pieces of lore throughout the games.
These special Heartless are called "purebloods," a distinction that has existed since the very first Kingdom Hearts title and held an important role with the original Organization XIII. As a result of the extra exploration of this lore, the real danger of the Shadows puts them high above the more common emblem Heartless that Sora fights throughout the series, even beyond their sheer numbers.
Implications of their Introduction
One of the strangest mysteries from Kingdom Hearts comes from how each of the three main numbered titles have opened with the Dive to the Heart segment on the iconic mosaic towers. The implication is that these towers are both the light inside of Sora's/Roxas' hearts, which are connected to other hearts that they will meet throughout the games. However, the fact that Shadows can exist even in the heart of Kingdom Hearts' goodest boy Sora implies that there really is nowhere that is safe from the pureblood heartless.
Emblem Heartless vs Purebloods
The difference between the emblem Heartless and pureblood Heartless is first mentioned in the Ansem Reports of the original title, but it is explored further in the Nintendo DS title Kingdom Hearts 358/2 Days that focuses heavily on Organization XIII. There are two major differences that separate these types of Heartless, one being the fact that emblems were created by the Ansem first introduced in the original Kingdom Hearts, and the second being that they purebloods don't steal hearts. Considering the extended lore behind being able to free and restore hearts, heartless, and nobodies that had eventually been introduced in Dream Drop Distance, this makes the pureblood heartless like Shadows even more dangerous.
Since pureblood Heartless don't steal hearts like their emblem counterparts, when a Shadow kills someone, there's significantly less hope of a victim being recompleted after being killed by the little enemy. Instead, while a Shadow wouldn't exactly be able to steal someone's heart, it would instead turn more and more people into more heartless, almost acting like a zombie army in how they multiply. Added to the implication that there will be less of a chance to be brought back to life by some Keyblade wielders like Sora, is the fact that the Shadows most dangerous feature is just how many there are.
Shadow Swarms
As anyone who's ever played any Kingdom Hearts before would know, Shadows never show up to a fight alone, instead choosing to fight in massive swarms to overwhelm their targets. Their large numbers also work perfectly together with the ability to sink into the ground, disorienting enemies as they pull focus in an unattackable state while more come to attack from behind. This also allows them to collaborate with other enemies, like the similarly hoard-like Soldier Heartless that act like the emblem versions of the low-level Shadows.
The huge numbers is most prevalent in the most recent title Kingdom Hearts 3, not only because the PS4 has allowed for more enemies to appear on screen at once than previous consoles, but because of the Heartless Swarms. These bundles of heartless took the form of the Demon Tower which Riku fights multiple times, and the sky-filling swarm that attacks Sora both in Twilight Town and in the Keyblade Graveyard. This swarm of Shadows in particular has even fueled a number of fan theories surrounding Kingdom Hearts Union X and how it and it's cast will continue to effect the future of the series.
Kingdom Hearts: Melody of Memory is set to release on November 13th, for PS4, Switch, and Xbox One.