Pharaoh 90 was an evil spirit who wanted to make Earth a second home from the Tau Nebula. In the Sailor Stars arc, it is revealed that Pharaoh 90 was one of the incarnations of Chaos itself.
Pharaoh 90 gained access to Earth through gene splicing experiments in Professor Tomoe's lab. A large explosion resulted in the destruction of the lab and the death of everyone with the exception of the Professor. Tomoe was distraught and wanted to save his young daughter, Hotaru. A black energy appeared and told him that it would save his and his daughter's lives under certain conditions. The Professor said he did not care about himself and that he just wanted to save his daughter. The black energy then said it would take over their bodies.
Pharaoh 90 had ordered Kaolinite to gather souls to him and to use bodies for Daimons. The main goal was to make Earth a second home from the Tau Nebula. When Mistress 9 awoke within Hotaru, she opened a portal to the Tau Nebula for Pharaoh 90 to draw power from. He merged with Mistress 9 and then merged himself to the Earth, infecting the land of the Omega Area, where Mugen Academy stood, and once the barrier around the Omega Area was broken, he began to spread around the planet. When Sailor Saturn awoke, she bound Pharaoh 90 and forced him from the planet, fatally damaging him, before sending him back to the Tau Nebula to die. Sailor Pluto used Dark Dome Close to seal the portal permanently, cutting the Tau Nebula off from Earth.
Phaoroh 90's a powerful dark force. It draws power from the Taioron Crystal and could communicate with them. It seems to hold a parasitic trait, like the Daimons, to infect a host and corrupt it, however on a much grander scale: instead of infecting a single human host, he tried to take possession of Earth to make it like the Daimon home of the Tau Nebula. Once he makes contact with Earth, is merged with his partner, Mistress 9, and draws power from the Tau Nebula, his darkness began to spread, allowing him to break the barrier the Outer Senshi formed around the Omega Area, and spread around the planet.