Soul essence in pure form.
Summary
Soul essence is a semi-physical substance which consists of pieces of sentient souls mixed with negative emotions. The aggregation has large magical potence. It is the distinctive resource used by the kingdom of Malekith.
Appearance
Soul essence appears as a faintly glowing white vapor with a pronounced purple hue. It swirls in vortex-like patterns when placed in containers, which are usually made of glass or a crystallic material.
History
Blurring the border between magic and technology, the concept of soul essence extraction was developed by the dark elves and is now a cornerstone of their society. Soul essence is many things: fuel, power supply, weapons, currency and even food. But it all started as a mere experiment. One priest of Arcanum who frequented the torture gardens in the Tantalize Reside once brought his sorcery equipment with him, and decided to study the effect of "external stimulation" on the soul rather than the body. His discoveries were surprising: When subjected to physical or psychological turmoil, am person's soul will "leak" substance, much like a cracked cup. This is the reason why extensive trauma is not easily forgotten – just as a cup needs time to refill, wounds need time to mend, both on the outside and the inside.
But what about the part of the soul that escaped through the cracks? When left alone, it swiftly disperses and escapes the tangible plane, but if timely gathered with specific intruments, it can be deposited in physical containers. Being a scholar of Arcanum, a religion that deals with souls and the afterlife, the priest instantly realised the magnitude of his discovery. The soul is literally the most powerful substance in the world, making the entire concepts of life and sentience possible. But because souls are metaphysical and can't be mined like coal or iron, the immence magical resources they represent remained untapped. The priest deducted that obtaining even a tiny fragment of that power through the atrificially extracted soul essence, as he named it, would turn the whole Malkian society on its head.
Application
After the Arcanum cleric revealed his findings to the royal council, research on a larger scale was set in motion, with the aim to uncover the full potential of Soul Essence, its applications, as well as an efficient way to produce it. As predicted, the substance turned out to be incredibly versatile. The first, and most obvious use of it is using it as fuel for spellcasting. Instead of sacrificing parts of one's own soul, any soul essence in one's possession could be used as a proxy to perform magic, without any direct consequences for the caster. Naturally, because a spell cast in this manner is no longer one's "personal" magic in the literal sense of the word, it takes much more intellectual skill and effort to shape an essence-using spell into desirable forms – it does not follow the caster's intuition at all. Soul essence has one large advantage over Muses as a magic source, however - it is portable.
The essence also found extensive use as industrial fuel. With the advent of vehicles and other technological innovations in other countries, Malekith found itself lagging behind, as it was very underdeveloped compared to other powers such as Alagos. Instead of trying to race its rivals to oil deposits, however, the dark elven research departments invested in the awesome potential of Soul Essence. They developed a type of device called the "essence capacitator" – a semi-mechanical, semi-enchanted crystallic tool that would bend any Soul Essence supplied to it for a specific effect. Provide essence to a Lighting Capacitator, and you have a lamp. Provide essence to a Heat Capacitator, and you have a baking oven. Provide essence to a Flight Capacitator, and you have an aircraft. Although infinitely versatile, such devices require massive knowledge of both magic and technology, as well as pin-point precision, in order to achieve exact results. In a sandbox, you have total freedom, and it is easy to use the sand and build something vaguely resembling a castle. However, building an authentic sand castle, complete with doors, windows and furniture, is a different matter entirely.
Being who they are, the night elves quickly got around to try and weaponize soul essence. They modelled special capacitator crystals which condensed the supplied essence into small, needle-like shards. With some mechanical additions, the massless shards could be propelled with great speed over short distances. Attach some ergonomical details, and voila, the first Splinter Rifle is born. The dark elven military scholars tirelessly continue their research – recent products include the long-range Soul Lance and the formidable Screaming Cannon.
Production
After the major areas of application have been explored, the next big question had to be confronted – where would Malekith get enough soul essence to meet its demands? Of course, the answer was rather easy – the already sinister torture facility of Tantalize Reside transformed from a mere amusement park into a heavy industrial complex. Outfitted to the top with the tools of the terrible trade, it houses vast amounts of unfortunate creatures who are doomed to spend the rest of their lives in endless agony, now not only to entertain their captors, but also to be milked for the invaluable soul essence.
Trade
Moderate quantities of soul essence are exported via the black market. The main buyers include foreign organised crime and tribal nations of demons, which do not have a set moral code and are not below taking advantage of the essence's power.
Use outside of Malekith
Because of its fundamentally immoral nature, the use of soul essence and any devices it runs is taboo in every other civilised society in Nyxheim. It is almost always punishable by law. Because it is very expensive outside of the dark elven internal market, only relatively powerful illegal organisations are able to get hold of usable quantities.