From Hero to Villain
Over 10,000 years ago, Zarkon was an ally of King Alfor of Altea and worked with him to create Voltron from a comet that fell from the sky before assuming the role of Black Paladin and Alfor assuming the role of Red Paladin to face of to an unknown life form escape from the rift. Together they formed Voltron with three others from different planets and destroyed the creature. After that, Zarkon fought at least one major battle and was able to use all of the Black Lion's abilities, After many years of experimenting with the rift, during which Zarkon married an Altean alchemist named Honerva, she and Zarkon grew obsessed with the power of Quintessence, as well as the possibility of it giving them immortality. Honerva grew ill during her studies and Zarkon tricked the other Paladins into going into the rift in an attempt to cure her, but they were both exposed to an overdose of the substance and the creatures they once fought. Presumed dead, Alfor evacuated the planet and gave orders to destroy it in order to close the rift forever. However, the two woke up, corrupted by the Quintessence, and rallied the Galra people in an invasion of Altea and the planets in their systems to claim Voltron for himself, but was deceived by Alfor into believing that the Lions had been destroyed. Despite this, Zarkon destroyed Altea in an act of revenge for their planet and killed Alfor
Haggar also found a way to infuse the quintessence into Zarkon himself, allowing him to remain alive and youthful over the next 10,000 years as he led the Empire in conquering most of the known universe. More recently, he would take a personal hand in the training of Commander Sendak, who became one of his most trusted soldiers. He would also discover that the Voltron Lions still existed, having been taken to safety by their former Paladins or hidden away by Alfor, and set out to reclaim them. This effort would result in the abduction of Takashi Shirogane, Samuel Holt, and Matthew Holt of Galaxy Garrison, whom Zarkon ordered to be interrogated.
Eventually, Shiro escaped Galra custody with the aid of the Blade of Marmora, a secret organization dedicated to Zarkon's overthrow, and made his way back to his home planet Earth, hiding place of the Blue Lion. He and a group of his comrades found the Lion and subsequently departed Earth in it, traveling to Arus, where Alfor had hidden the Castle of Lions. When Haggar sensed the awakening of Allura, Alfor's daughter, Zarkon contacted Sendak and ordered him to travel to Arus and deal with the threat she posed and capture the remaining Lions. Unfortunately for Zarkon, the Paladins succeeded in recovering all five Lions and forming Voltron, destroying Sendak's ship through the commander and a handful of his troops survived. Haggar soon turned her attention towards the creation of a Robeast to deal with Voltron, though after an infusion of quintessence Zarkon felt he had the power to crush the new Paladins himself.
When Haggar's warrior Myzax fell before Voltron, Zarkon contacted Sendak on Arus and instructed him to capture the Castle of Lions in order to redeem himself, and was confident in his ability to succeed. Sendak came close but was thwarted, and the Castle of Lions departed Arus and made its way towards a Balmera that the Galra had occupied and mined for its powerful crystals. When Commander Prorok suggested mobilizing the Galra fleet to seize it, Zarkon chose instead to heed Haggar's counsel and remain in place while she conducted an experiment with a new device with an even greater ability to harvest quintessence. When Prorok later came to report that he had a fleet ready to capture Voltron, Zarkon dismissed his claims as foolishness, being far more acquainted with Voltron's powers than his disobedient underling.
Soon, Zarkon received a pleasant surprise as Allura, who had been captured while infiltrating a Galra ship, was brought to him, and when she claimed that Voltron would destroy him assured her that it would only make him more powerful. When the Paladins formed Voltron and attacked his command ship in an effort to free her, Haggar used her magic to boost Zarkon's connection to the Black Lion, allowing him to seize control of it. When Allura's aid Coran disrupted the effort to tractor the Black Lion aboard the command ship, Zarkon flew out to retrieve it himself only to be engaged by Keith, Paladin of the Red Lion. Revealing his possession of the Black Bayard, Zarkon easily held Keith at bay with the weapon but was then surprised when Keith unlocked a canon ability of his Lion. The continuing struggle prompted Zarkon to remark that Keith fought like a Galra, but he nearly killed the young Paladin before being halted by Shiro, who had reclaimed the Black Lion; the Paladins then made their escape.
Having determined that an inside agent enabled Voltron's escape, Zarkon accused Prorok of treason and turned him over to Haggar before appointing Thace to the rank of commander and assigning him to root out any other possible traitors. Zarkon then turned his attention to hunting down Voltron, with the Druids strengthening his connection to the Black Lion to enable him to locate it. Their first success in doing so was followed by Prorok-now a Robeast-being sent to intercept Voltron, only for him to be destroyed by the Blade of Marmora agent Ulaz using gravity warping technology that Zarkon wished to possess. As a result, Galra Central Command came after the Castle of Lions, intercepting it several times shortly after its departure from Olkarion. Zarkon's determination to pursue Voltron to the exclusion of all else prevented the Galra from seizing an opportunity to destroy the Castle of Lions, which was able to escape from them yet again.
Zarkon's ability to locate them caused the Defenders of the Universe some consternation and briefly led to Keith and Allura leaving the group. After their departure Zarkon sensed the Black Lion on Taujeer and directed Morvok to capture it, ignoring the sniveling commander's attempts to weasel out of the assignment. Morvok's failure allowed Voltron to escape yet again, while also making them aware that Zarkon was using the Black Lion to locate them. This led to Shiro entering a realm of thought accessible to the Paladins of the Black Lion, where Zarkon sensed him and attacked in a bid to crush his rival once and for all. However, while Zarkon's greater experience proved too much for Shiro, Shiro's respect for the Black Lion, as opposed to Zarkon's lack of regard, resulted in the Lion coming to Shiro's aid, leaving Zarkon's connection weakened.
Determined to regain the Black Lion before his connection was entirely lost, Zarkon also put Haggar to work on a means of enabling him to combat Voltron himself if necessary. Zarkon became increasingly irritable and obsessive as time went by, ignoring even Haggar's counsel in his mania to locate the Black Lion and recover it; he would also ignore obvious danger signs when the Defenders of the Universe attacked Galra Central Command. With the vessel disabled and transported to another part of the galaxy, Zarkon entered his new battle armor despite the fatalities of previous pilots and faced off with Voltron. A furious battle ensued, during which Zarkon reflected the Castle of Lions' firepower back at it and engaged both Voltron and its individual Lions in combat. However, after Shiro succeeded in recovering the Black Bayard, Voltron launched a final attack against Zarkon that left his armor all but destroyed.
Zarkon was left in critical condition, with quintessence being routed into his comatose form; Haggar thus directed the Galra Commanders to summon his son, Prince Lotor.
Zarkon is a ruthless conqueror who believes that the strong have the right to rule and survival, while the weak must perish. He has made this the creed of the Galra Empire, as well as the fanatic maxim that "Only success or death will stop" individual members of the Empire. Zarkon has no patience for flattery or for disobedience and is willing to have even veteran commanders disposed of in the event of failure. Haggar is Zarkon's closest confidant, and usually the prevailing voice in any council between Zarkon and his command staff.
Zarkon's greatest weakness was his obsession with reclaiming possession of the Black Lion, which he saw as his rightful property and as a weapon answerable only to him. In this respect, he failed to recognize that the Black Lion possessed a will of its own, something that Shiro was fully aware of and embraced, which strengthened the human's connection to the Lion and weakened Zarkon's. Zarkon's determination to capture the Black Lion blinded him to all other concerns, and even Haggar was unable to dissuade him when he was caught up in it.
Zarkon was incredibly powerful, having spent the past 10,000 years being infused with quintessence that allowed him to maintain prime physical condition and may have had additional benefits as well. His resilience was incredible as he was not only able to pilot his battle mech-which had killed all previous pilots-but to survive its destruction by Voltron's Blazing Sword, though he was left comatose as a result. Though the vastness of his empire forced him to rely upon his commanders to oversee distant operations, Zarkon was no stranger to battle on the front lines. His usual attire consisted of a suit of armor to which his royal robes and ceremonial armor could be attached or detached, and incorporated a jetpack that made him capable of flight.
While in possession of the Black Bayard Zarkon could use it in a variety of forms and functions, whereas the current Voltron Paladins have only been seen to demonstrate one weapon produced by their Bayards. This weapon was capable of striking down one of Haggar's Druids with a single blow and also enabled Zarkon to face the Red Lion in combat with no other armaments or support. Zarkon's connection to the Black Lion also allowed him (with Haggar's aid) to locate it even from vast distances away, and at close enough range he could seize control of it even while Shiro was in the cockpit or the Lion was part of Voltron. He could also enter an astral plane accessible to the Black Lion and its Paladins, and proved a skilled combatant who nearly succeeded in killing Shiro. However, their battle in that arena resulted in Shiro's connection to the Lion growing and Zarkon's weakening, which left him unable even with aid from Haggar and the Druids to locate the Black Lion unless Shiro broadcasted his location to Zarkon.