Queen Diana was the ruler of the Moon--or as it was called then, Luna--in the age before the Silver Millennium. After hearing a prophecy from the powerful oracle Nehellenia that she would come to rule the entire solar system, she allied the planets with her using intimidation. The Moons, unwilling to accept her rulership, resisted Queen Diana and were destroyed for it.
Although Queen Diana was Queen Serenity's mother, she was long dead by the time Princess Serenity was born. There are few records of the time that she ruled, saying only it was a peaceful era when the moon came into power--and there are no records of how she met her death at all, save that she did, in fact, meet an untimely death.