Quintessence is a substance with the highest known energy per unit volume in the universe. Raw quintessence is capable of being extracted from planets and refined into a standardized fuel for Galra technology.
It is heavily implied that Zarkon's unnaturally long lifespan as well as his immense physical strength is due to regular infusions of refined quintessence.
Throughout the first season of Voltron: Legendary Defender, we see multiple references to the substance. The most common is Zarkon levitating between numerous spires that blast the energy at him, and he hints that this is what has given him his long lifetime.
The Galra Empire takes over planets in order to harvest Quintessence, and they find that the crystals that Balmera naturally produce can contain quintessance. The Galra use mining shafts to dig deep into the Balmera and harvest the crystals. Unfortunately, this leaves no room for the Balmera to gain something in return, and thus, the only seen Balmera, X-95-Vox, was severely damaged when the Galra mined into it.
Later in the first season, Haggar is able to create a system called the Komar Experiment that sucks the quintessence directly out of a planet, leaving it completely barren of any substance. Zarkon remarks that this will allow them to quickly gain a massive amount of quintessence, and not need to physically conquer the planet for sustenance.
- The idea of Zarkon extending his lifespan with substances was touched upon in the original Voltron series, where Zarkon briefly mentioned using the flowers from the planet Lyra to extend his already long life. 
- Per Wiktionary: Etymology: Middle English, from Middle French, from Medieval Latin quinta essentia "fifth essence".
- Noun: quintessence (countable and uncountable, plural quintessences)
- A thing that is the most perfect example of its type; the most perfect embodiment of something; epitome, prototype.
- A pure substance.
- The essence of a thing in its purest and most concentrated form.
- (alchemy) The fifth alchemical element, or essence, after earth, air, fire, and water.
- (physics) A hypothetical form of dark energy postulated to explain observations of an accelerating universe.
- Infusion with a living being seems to cause emaciation and gradual insanity of the user as Haggar as well as her cat seem to have drastically lost weight and suffered radical changes in personality.
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