Pharaoh 90 is the main antagonist of Sailor Moon S, and the one behind all the events.
Pharaoh 90 has the appearance of a giant, dark energy orb.
He had Germatoid possess Doctor Tomoe, and his most powerful servant, Mistress 9, the Sovereign of Silence, possess Tomoe's daughter Hotaru. Mistress 9 and the Bureau of Bad Behavior wanted the Purity Chalice to awaken him, but Sailor Moon got it first.
Eventually Mistress 9 was able to get the Purity Chalice and awaken Pharaoh 90, who came to Earth. Sailor Moon attacked him, but he was unaffected and blasted her with a red energy wave. He also shot an energy wave at Mistress 9, but Sailor ;Moon protected her, as killing Mistress 9 would kill Hotaru as well. Pharaoh 90 began to bring the Silence about the world to destroy it. Hotaru was able to overpower Mistress 9 from within, destroying her, and awaken as Sailor Saturn. She attempted to destroy Pharaoh 90 from within by destroying his core, but was unable to. Sailor Moon received power from the other Sailor Scouts, transformed into Super Sailor Moon without the Purity Chalice, entered Pharaoh 90, and destroyed his core, completely destroying him.
In the manga, Pharaoh 90 commanded Magus Kaorinite to collect souls from people of Earth to sacrifice to him, and use the bodies as hosts for Daimons. Eventually Mistress 9, a Daimon who resided of Hotaru, took control of Hotaru's body and took Chibi-Usa's soul and her Silver Crystal. Mistress 9 uses the power of the Silver Crystal, and opens a doorway into the Tau Nebula. After Hotaru breaks free of Mistress 9, Mistress 9 merges with Pharaoh 90, allowing Pharaoh 90 to merge with Earth and became more powerful once the portal to the Tau Nebula was opened and he could turn Earth into a second home for him and his Daimons. However, Hotaru awoke as Sailor Saturn, severely damaged Pharaoh 90 and forced him from the Earth and back to the Tau Nebula. Sailor Pluto closed the portal to the Tau Nebula forever, ensuring he could never return to Earth again.
He has no defined appearance in the manga.