Prince Diamond acted under Wise Man's guidance the whole time. Diamond loved his younger brother Sapphire. He tried to stop Wise Man from killing him but Wise Man did not listen. He has since grown wary of Wise Man and promised himself that he would settle the score of his brother's death after his revenge was complete.

Aside from Diamond's dark powers, the Black Moon insignia on his forehead could also turn into a third eye that could mesmerize people. (See our animated gif gallery for a sampler)

Diamond soon realised that Sailor Moon was the woman he coveted in her younger days. He succeeded in catching her a few times and tried unsuccessfully to make her his... (Tuxedo Mask would always save her at the last moment...).

With Sailor Moon's help, Diamond was able to see the truth in the end -- Wise Man had been using the whole Black Moon Clan for his own purposes the whole time. Wise Man commanded Diamond to kill Sailor Moon but he refused. Ultimately, he and Wise Man fight each other. He could have won but he sacrificed his life to save Sailor Moon. With his last breath he tells Sailor Moon to take care of the Black Moon sisters living on Earth, and that he is glad there is still hope for his clan.