The Dark Kingdom - the first enemy that the senshi had to face. From way back in the Silver Millenium, an ancient evil named Metallia longed for release. However, the powers of its ally Queen Beryl were not enough to release it; even though Queen Beryl collected energy from hordes of humans and gave it up to Metallia, it was not enough. Metallia needed to have the formidable powers of the legendary crystal, the ginzuishou. Only then would she have the power to free herself.
Queen Metallia is really little more than a shadow with a nasty expression - and yet this featurelessness makes her all the more frightening. Evil is easier to face when it has form; Metallia's very amorphousness makes her somehow more menacing. Long ago, Queen Serenity and the people of the Moon sealed her away. Her one goal is the ginzuishou; it contains the power that can free her. Even the attacks of all the senshi together didn't faze her; she merely absorbed the energy. Ironically, it was the power of the ginzuishou that finally destroyed her.
You've got to feel sorry for Queen Beryl. Just a little bit. Queen of the Dark Kingdom, Beryl has it all - lush beauty, dark powers, and dominion over an entire dimension. And yet... Rejected by Prince Endymion, this bitter young woman named Beryl sold her soul to the demon Metallia and became Queen of the Dark Kingdom. Metallia directed Beryl to get the ginzuishou. However, Beryl was not without ambition of her own. She knew that if she acquired the ginzuishou, she could use it for her own benefit -- gain youth and longevity, and unparalleled strength. So even as she mouthed promises to Metallia, she pursued the ginzuishou for her own purposes. She never really gave up her love for Endymion, either. She chased after both the prince and the crystal with single-minded intensity. She was doomed to failure; in act 11, Sailorvenus ran her through with the holy blade of the Moon. (It was justified; at that point, she was trying to strangle Usagi with her hair.) Beryl's last words were words of longing for Endymion.
The four Earth generals... Jadeite, Nephrite, Zoicite, and Kunzite are very handsome and powerful. When the series begins, it seems that they worked for Queen Beryl - but she had been controlling their minds all along, and they're actually good guys deep down.
This didn't stop them from being extremely effective. Jadeite almost killed Rei before Sailormoon stopped him; Nephrite was equally effective in trapping Makoto; Zoicite was seconds away from killing Sailormoon before Sailorvenus killed him, and Kunzite managed to kill Tuxedo Kamen before the senshi combined their powers to evaporate him. Kunzite actually managed to overcome Beryl's control of his mind for a short while, but she ruthlessly took over his mind again.
After their bodies were destroyed, the generals were able to do in death what they were unable to do in life. Their spirits remained with their master Endymion to protect him, and he called upon them frequently whenever he needed advice.
One of the most horrifying things that Queen Beryl ever did was to reanimate the corpse of Tuxedo Kamen, and fill its mind with evil. Evil Tuxedo Kamen is recognizeable by his dead black eyes. Normally, Takeuchi-san's characters eyes sparkle with emotions, giving them personality; however, the eyes of Evil Tuxedo Kamen are emotionless and blank.
After the senshi killed Kunzite, Queen Beryl animated the corpse of Tuxedo Kamen and sent it to work for her. He was the most effective of her tools; he didn't even flinch from the senshi's attacks. He pursued the senshi with maniacal glee, not hesitating to attack with full force. Besides which, Sailormoon was reluctant to attack the creature that wore the body of the man she loved. In the end, Sailormoon couldn't bear to kill him without killing herself as well - which she did. (She flew up to him, drove the holy blade of the Moon into his chest as she kissed him, and then turned and thrust it into her own heart. It was very sad.)
Anyway, after Sailormoon and Tuxedo Kamen died, the senshi threw themselves against Metallia and were killed. The power of the ginzuishou reincarnated them all -- but while it could bring Mamoru's body to life, the contamination of evil was still evident in his black eyes. He had control of his body and actions once more, but he was blind.
Sailormoon drew on the ginzuishou and the full power of the Moon, and blasted Metallia away. Then, having exhausted her resources, she fainted. Mamoru lifted her body and kissed her on the lips. She was awakened by the kiss of her prince, and smiled at him. He lifted his head from hers... and his eyes were clear.
Isn't that sweet? True love conquers all. Very reminiscent of Snow White at the end, except that the prince got something out of it too.