It is no secret that the species Mercury belongs to survive by feeding off each other's emotions - one's smile can quite literally keep another going. In the case of Mercury, he feeds off affection and (his partner's) pleasure of whatever variety - this, also, is no secret.
What used to be a secret for the longest time, however, was devising a means to "store" these emotions for use as energy supply or personal "rations" to make solitude significantly less lethal to his species. That is, used to be.
A technological breakthrough by Mercury himself (aided by some of his closest friends, of course) has resulted in a way to store emotions of any variety, utilizing books fabricated with unique pages. By slotting vials with empty "crystals" into the spine and spilling one's thoughts in any form - whether writing or drawing - it is possible to extract a particular color of emotion and effectively infuse the crystal with it.
This energy is known as Ego, and the crystals to store it become Shards of Ego (or Ego Shards, if you're less poetically inclined). Once infused with a particular emotion, an Ego Shard can be used to power certain emotively-responsive technologies, liquidized for more specialized application, or the infused Ego can be taken into oneself as an organic "substitute" for contact with others to prolong one's life (though the mental consequences of extended time away from others are as yet unresearched; it is still advised to return to civilization as soon as possible).
Now, these Ego Shards contain raw, unbridled emotion - damage to the structural integrity of the crystal would have catastrophic consequences no doubt, likely manifesting in a different manner based on the color of the Ego within. Fiery Ego - representative of anger - would almost certainly cause physical harm upon any and all nearby individuals and structures; Pink Ego would perhaps act as a wide-area aphrodisiac dispenser. As is to be expected, noone is quite willing to test what would happen if an Ego Shard were to fracture.
What is known however is that a particular Ego Shard can only hold a mixture of so many emotions and voices within - the books used to extract Ego typically ensure where they can to seperate different colors into individual Shards, but if too many conflicting viewpoints, too many people and emotions flood into the same crystal, the harmony within can quickly become a cacophony and a threat to the integrity of any living creatures. This Cacophony Shard, and all its voices within will begin to panic and frantically seek other voices to add to its song, quite literally crystalizing parts of anyone unlucky enough to be nearby and splintering their self in the process, distilling the sum of their emotions into one large, sapient Ego Cluster, similar in form to a crystal golem. The process is reversible, albeit painstakingly slow, as the crystals composing one's person must be carefully chiseled off to reveal the original form underneath, then one's emotions must be gradually fed back into themselves - too much at once, and they will run rampant with that emotion. As a result, people having undergone such a transformation will typically bear at least a small sliver of crystals along their body for a very long time, and it's even known this will impact their attuned element somewhat, granting them the ability to create crystals from their natural magic, even those that do not "wield their Heart".
These people are known as Egogeotics, and will gain "Ego" as an additional attuned element.
This piece was originally entirely unrelated to my worldbuilding - my friend Glitched-Corruption on deviantART wanted to include Mercury in her Spooktober prompts, and while I'm not one for horror, the prompt of "Crystalline Overgrowth" piqued my curiosity, as I'd been developing Ego at the time already. She decided to go the extra mile and create a first concept of the "Ego Journals" (name pending) as well, and I'm simply amazed by the result. She says she's not good at art, but look at this and tell me she isn't.
Also, yes, the title is shamelessly stolen from Final Fantasy XIV: Shadowbringers.
