Master Pharoah 90 is the spirit and leader of the Tau star system. Like the other arch-enemies in the Sailormoon universe, he has no specific shape and little form; he is a swirling field of stars, accompanied by a smoky scent of death. When his system exploded, he went searching for a new home. He found it in Earth. He intended to assimilate this planet, full of life, into his own self, changing it into what used to be the Tau system.
Earth was already inhabited by people, but Pharoah 90 didn't really mind. He subverted people on Earth to do his bidding and feed him power; the power that he fed on were the souls and life-essences of humans. He let his human servants draw on the power of the Tau system crystal, the Taioron Crystal, to do their work.
The most useful of his human servants was Souichi Tomoe. Professor Tomoe founded Mugen Gakuen, the elite school that drew many students. These unsuspecting students provided energy that fed Pharoah 90, and their bodies became vessels for daimons.
Another of his powerful servants was Kaolinite, who commanded the Witches 5 against the senshi. The humans communicated with the Master through the means of a reflecting pool; his star-bright eyes would appear in the water and his voice would be heard throughout the room.
However, Pharoah 90 couldn't actually reach the Earth. Only one creature, Mistress 9, had the power to open the doorway for him. So Pharoah 90 gave Tomoe a daimon seed containing Mistress 9; Tomoe installed it inside his own daughter, Hotaru. Pharoah 90 waited patiently, absorbing more and more of the energy sent to him by his servants, until Mistress 9 took over Hotaru's body.
Mistress 9 also managed to acquire Chibiusa's Silver Crystal; with the addition of this new power, she was able to open the doorway for Pharoah 90. He appeared in the sky, a galaxy-shaped swirl of stars, heading towards Earth. Then Sailorsaturn appeared, called by the three talismans. She forced Mistress 9 and Pharoah 90 back through the doorway. Sailorpluto raised the Garnet Orb and shut the doors forever. The Earth was safe once more.
It must have been a very annoying experience for Pharoah 90. He came incredibly close to taking over the Earth - just to be thwarted at the very last moment.