Millennian's solar-powered spaceship crashed on Earth during the Cretaceous period, and they remained dormant for ages until drilling operations exposed them to sunlight again, waking the sunken menace. They initially plan to modify Earth's atmosphere to suit their needs as well as giving cryptic message to humanity that they would conquer Earth, but with Godzilla stood on their path and discovered that the kaiju's genetic issues can restore their antimatter converted bodues, they decide to instead modify themselves using genetic material from the monster, hoping to acquire its regenerative abilities and restore their bodies that reduced into antimatter.
After some difficulties in incapacitating the kaiju, the ship's crew managed to absorb and use Godzilla's genetic samples to restore their bodies, and manifest themselves as a huge tentacle creature somewhat resembling a mixture of jellyfish and mushroom. Though they successfully restore their physical form as a composite creature, something webt wrong: As Yuji Shinoda stated, Millennians were overestimate Godzilla's Organizer G1 in his genetic material, and as result, the composite form of Millennians wind up mutate uncontrollably as they helplessly watch their composite body taking some traits of Godzilla, giving rise to the hybrid monster known as Orga that was immediately dispatched by Godzilla himself.
While Millennians have malicious intention to conquer Earth, they dobnot dependant upon brute force in doing so, as they seem to want to gather all the information from technology that they can to carry out such task, via telepathy, hacking, and biological information. This is apparent as when they attacked a news building, they hacked every single computer in that building to find out everything on Earth including Godzilla, whom have Organizer G1 that they need to restore their bodies. They also seem to not want to fight unless absolutely necessary; even once it has mutated into Orga, it is obvious that it does not want to battle Godzilla until it realizes that escape is not an option.
It is unclear what a Millennian's actual form looks like, but in Godzilla 2000: Millennium, at first, it looked like a ball of glowing yellow dust of antimatter when exit the ship. Then, after absorb enough genetic material from Godzilla, they immediately accessed their composite form that looked like a huge tentacle creature somewhat resembling a mixture of jellyfish and mushroom.