In the Voltron: Legendary Defender universe, the Altean race is of significant historic note, and even with few members left, this race still helped shape the fate of galaxies.
The Alteans had their heyday 10,000 years ago when their civilization was at its peak. Back then, they pursued peaceful areas of interest ranging from shipbuilding to arcane alchemy and more. But the Galra Empire tried to wipe them out, leaving few Alteans remaining. The five Paladins of Voltron gained a few insights to the Altean people and fought hard to protect the survivors.
Who Are the Alteans?
A few flashbacks and many explanations from Allura and Coran reveal to viewers what the Altean people are like. In the old days, before the planet Altea's destruction, these humanoid aliens lived in a peaceful and highly advanced civilization. By many standards, it was an ideal civilization, since Altea was a place of bountiful natural beauty and advanced technology alike. The Altean people did not believe in conquest and were semi-pacifistic, but they still had a formidable fleet to their name.
They placed some value on martial prowess, though, regularly training soldiers and officers to defend Altean interests from aggressors. The Alteans were not a warrior culture overall but they did venerate war heroes. In fact, their warships were some of the most advanced and powerful of their time, complete with particle barriers. The Alteans also trained with small arms, such as swords and polearms. In close combat, they could fight much like human warriors would in later millennia, having humanoid bodies and similar physical strength and capabilities.
But the Altean people placed a far greater emphasis on the pursuits of science, technology and alchemy. This was the true foundation of their civilization's prominence. In particular, the Alteans alone had access to Oriande, a mythical and hidden place that could grant any worthy visitor the secrets of advanced alchemy, allowing them to perform feats on par with magic itself. The white lion guardian protects those secrets, as princess Allura later finds out.
Altean science led to the creation of a portal to a quintessence field on Daibazaal (the Galra homeworld), and such material was used to forge the Lions of Voltron. This led to the Altean civilization's greatest defense, and the five Lions were piloted by Paladins of differing species (including Zarkon). However, this ultimately led to the Altean people's doom.
The Altean Genocide And Resurgence
Emperor Zarkon of the Galra Empire -- once a friend to the Alteans via King Alfor -- became corrupted by the Altean experiment with quintessence. Zarkon's wife, Honerva, got sick from the quintessence, and both of them became tainted from extreme exposure. Worse yet, Zarkon's homeworld, Daibazaal, was destroyed by the portal being enlarged, and Zarkon wanted revenge. He made sure to destroy Altea, wiping out almost every Altean. In response, the Alteans scattered the five Lions across the universe to protect them from Zarkon, while a few thousand colonists living off-world were spared as well. Princess Allura and Coran survived in the Castle of Lions, emerging from cryosleep when the soon-to-be Paladins discovered them.
Allura, as the last surviving royal Altean, quickly formed the Voltron Coalition to unite the oppressed races under Voltron's banner. She also briefly worked with Lotor until another Altean, a colonist named Romelle, warned her about Lotor's scheme to harvest quintessence from unwitting Altean colonists. Allura, who knew the secrets of Oriande and Altean alchemy, turned on Lotor and helped Voltron defeat him once and for all, and she also helped lift the siege on planet Earth sometime after that.
Haggar, Zarkon's wife and aide, abandoned her husband's scheme to rule the Galra Empire and instead turned her attention to her fellow Alteans. She wanted to rebuild the old Altean civilization, and so she recruited the Altean colonists and corrupted them. They fought for her in giant robots that could match Voltron in strength, and in her desperation to reunite with her family and get her happiness back, Honerva/Haggar tried to travel through parallel universes. Doing so nearly split all of reality apart, but Allura made an emotional appeal to her. Allura and Honverva both used Altean alchemy to give their lives and heal all of reality. Honerva got her happy ending when she found a peaceful reality with her husband and son to live in. All threats to the Altean people had ended, and the surviving Alteans were able to rebuild their civilization in a peaceful universe at last.