The Noble Guardians are a collective term for the legendary trio of the Tesla Region: Neonious, Kryptus, and Xenious. All three are share a similar poison typing and are based off of the Noble Gasses on the periodic table.
Neonious and Kryptus were created by Arceus to act as untouchable warriors due to always being a noble gas. They were victorious in many wars and were considered invincible by kingdoms across the regions, until Neonious and Kryptus were given the ability to think freely even though being lifeless. This caused them to turn on Arceus and create their own group called the Noble Guardians, led by Neonious, remaining neutral only with each other. The Noble Guardians slowly grew an army and faced deadly battles. During a war against Arceus, the Noble Guardians were captured and imprisoned into chambers that were meant to never be opened again. Sealing their evil energy away forever.
Thousands of years later, Tesla scientists create a third guardian with Xenon to replicate and better understand the Noble Guardians. The third guardian was kept in depths of the Northern Labs never to be used again after histories gave warning to the destruction it could cause.
After beating the two Guardians, their souls are absorbed by the artificial Xenon guardian, Xenious. The mixture of their souls created a new guardian altogether.