As with all Celestials, little is known about Ziran the Tester. But Like his fellow Celestials, Ziran stands at 2000ft. tall, and possesses a near limitless range of powers and abilities. Ziran's role is to carry out the testing process on whatever species or civilizations that the Celestials choose to conduct their genetic testing on.
Ziran has been a part of the first, second, third and fourth Celestial hosts on earth, although he was not present during Arishem's cout d'etat, where-during the Celestials second host to Earth- he and 4 other Celestials (Nezzar the Calculator, Hargen the Measurer, Jemiah the Analyzer and Tefral the Surveyor) attacked and betrayed Tiamut the Communicator(also known the Dreaming Celestial) and after a protracted battle that shaped and carved the Earth into what it is today, Tiamut was overwhelmed and sealed inside one of Earths mountains.
During the fourth Celestial host on Earth, Odin infused his life force as well as that of all of Asgard(save for Thor) into the near invincible Destroyer Armor, and alongside the Uni-Mind(which was a collective of all the Eternals combined into one being for the purpose of engaging an enemy that no single Eternal could) confronted the mighty Celestials. However the battle was brief, as Ziran and his fellow Celestials eliminated the Uni-Mind with a single blast of energy and reduced the Destroyer Armor to molten ruin minutes later.
Ziran was briefly seen when the Beyonder visited the Celestial homeworld, as well as the birth of an infant Celestial(only known as the Blue Celestial) in the Black Galaxy. It was revealed that the Black Galaxy had in fact been a Celestial experiment, with the entire Galaxy itself comprising the Blue Celestials brain. It was never acknowledged as to whether or not this was how all Celestials are birthed, or if this was just an individual experiment that the Celestials had conducted. When Thanos acquired the Infinity Gauntlet, Ziran was one of two Celestials(the other being the One Above All Celestial) along with Adam Warlock, Galactus and many other cosmic powers of the universe attempted to oppose him and strip him of the Gauntlet, and despite their best efforts, their immeasurable combined might was as nothing compared to the power of the Infinity Gauntlet. After Thanos was defeated, and Adam Warlock gained the Gauntlet, Ziran the Tester was present when Eternity petitioned the Living Tribunal to strip Warlock of his new power, however Ziran was only tasked as observing the hearing, and nothing more.
When Thanos had obtained an even greater power than the Infinity Gauntlet(the Heart of the Universe itself) Thanos destroyed Ziran(to the shock and awe of Galactus himself) followed by the entire universe, although Thanos quickly realized his folly, and restored everything back to the way it was, leaving no memory of the events that had occurred.
It was revealed that Thanos was a product of one of Ziran the Testers many genetic experiments.