Who are the Heart-Eaters?
They're an Apocryphal/Optional type of Exalted available in the Exigents book in 3e. Unlike the other two optional Exalts (Dream-Souled and Umbral), they're not optional because they might or might not exist as the ST chooses, but rather whether they are even in play or not. No matter what, they DO exist because of how important they are to the Sovereign's origin story.
Back during the Primordial War, there was an 8th Celestial Incarna who helped with the Divine Revolution: Aurora, the Lord of Muses, God of the Rainbow, etc. He had his own Exalted known as the Aurorals who mostly were supporter-types. They had the unique ability to tie another Exalt 's Anima Banner to themselves and duplicate the powers of it, greatly improving both of their abilities. They were also artists who greatly improved the armies morale.
Until one day in the war, Aurora himself was caught by the Primordials up in the Northwest and killed. THEN, they tore out his heart and inflicted the worst curses ever imaginable onto it (well, 2nd worst after the Great Curse, but that dying energies to it to give an extra oomph). Those curses then infected the Aurorals thanks to the heart being a point of sympathy, corrupting them into hive-mind monsters known as the Heart-Eaters.
The Primordials expected the Heart-Eaters to betray their former allies and serve them, but they actually didn't. They still fought on the side of the Gods, getting A LOT of side-eyes on the way, and were there when victory was eventually declared. Peace was established in the land... And it crashed against the Heart-Eaters newfound instincts. They had an addiction to mind-controlling people, because it was the only way they could positively self-reflect, so they kept on taking more and more Pawns. Eventually, the other Exalted decided that enough was enough, separated them from their Pawns so that they couldn't use them as a respawn point, killed them and scattered their opal bones deep into the Wyld, never to be returned.
...Except when the Raksha encountered some of those bones, decided to get a full collector's set, and brought them closer to Creation over the ages. Whenever a mortal touches the complete set of opal bones, their soul clashes with the trapped spirit insides the skeleton, typically creating a new personality that becomes the new Heart-Eater. Currently, there's anywhere from 0 to 20 Heart-Eaters running around at the borders of Creation at any given time. Many are clustered around the Northwest, because that's where Uluiru and the Fount of Glories.
The Fount of Glories that uses the blood of their Patron for Exaltation. Opinions vary on what to actually do with it, but they unanimously want to pry it from the cold dead hands of the Sovereigns and use the Fount in some way to benefit themselves. Some believe that it can cure them of their possession addiction, which is...
probably not true. Or if it does, it might only work for 1 Heart-Eater and then ruin the Fount for anyone else. No one knows.
EDIT: Added this to the Informational threadmarks, because it seems useful.