We find out late in the first season that the Dark Kingdom as led by Queen Beryl was comprised of warriors from Earth. So, although many of the other enemies in the series were aliens, the Dark Kingdom/Negaverse is mostly made up of people who were originally human. In the manga, we discover that Queen Beryl was once a normal woman (who perhaps had dabbled in sorcery) in Prince Endymion’s court, and had an unrequited love for him that became intense jealousy over Princess Serenity. There is some semblance of this plot point in the anime, as in the flashback to the final day of the Silver Millennium, Queen Beryl makes an offer to Endymion that they should marry and rule both the Earth and Moon together. In the present day, she seems to enjoy the idea of his brainwashed self being entirely subservient to her. At any rate, it is because of her jealousy that Metalia is able to corrupt her heart, and Metalia grants her powers that allow her to gradually turn all of Endymion’s subjects against him. Beryl shows on multiple occasions that she fears Metalia’s power and wants to conquer her, but also reasons that it’s much too late to turn back.
In the Silver Millennium battle in the anime, Queen Beryl was sealed by Queen Serenity’s Silver Crystal, and she and the other members of the Dark Kingdom were sent to Earth with the others. In the manga, Beryl kills Endymion after Metalia possesses her. Princess Serenity commits suicide in grief, and Venus stabs Beryl with her Holy Blade, knowing it is too late to win the battle. History repeats itself in the present day, as it is once again Venus who slays her with the Holy Blade after she and Jupiter free Sailor Moon from Beryl’s attempted choking of her.
You can also spell her name as “Metallia,” but “Metaria” is incorrect since most of the villain names in the series are derived from gemstones, minerals, alloys and metals. The English dub does not give her a name, instead Queen Beryl only refers to her as the “Negaforce,” which is very confusing (especially since it sounds very close to “Negaverse.”)
Unlike the other members of the Dark Kingdom, she was a malevolent alien being from the sun. She has no humanoid form like the others, instead she appears mostly as a black force of energy. Queen Serenity explains that because she was so grief-stricken, her seal on Queen Metalia was incomplete. When Metalia’s seal broke, she was able to resurrect the Dark Kingdom. In the manga, Beryl was reincarnated as a normal woman who encountered Metalia in the Arctic Circle. Metalia restored her memories and granted her the power to resurrect the Shittenou. Her seal in the anime kind of makes her look like a lava lamp. We find out at the end of the manga that all of Sailor Moon’s most powerful enemies (Metalia, Death Phantom, Pharaoh 90 and Queen Nehelenia) were aspects of Chaos, the origin of evil in the universe.
In the anime, it was Metalia who killed Princess Serenity and Prince Endymion by blasting them with an enormous amount of energy. She is the real leader of the Dark Kingdom, not Beryl, since Beryl gets almost all of her powers from Metalia. In the present day, she lacks enough power to truly break free of her seal, and commands Beryl to find the Silver Crystal so she can use its power. Near the end of the season, we see that her power is growing because there are sunspots appearing on the sun (and they’re causing darkness to spread over the Earth). In the manga, Metalia had hoped to find the Silver Crystal before the Moon Princess awakened. The final battles in the anime and manga differ greatly. In the manga, she grows large enough to be able to absorb a seemingly dead Sailor Moon, Tuxedo Mask and the Silver Crystal. They instead awaken inside Metalia, and Sailor Moon is able to become the true Princess and activate the Silver Crystal’s full power, sealing Metalia for good.
People can be forgiven for not knowing that this is the term for referring to the fusing of Queen Beryl and Queen Metalia, since it’s never actually mentioned in the anime, just in supplementary material. Most of the scenes leading up to their merging were cut from the English dub, but after Sailor Moon heals Prince Endymion, he throws a rose into the crystal shard that Beryl throws at the pair. It shatters, and at the same time the shards are hitting him in the back as he’s protecting Sailor Moon, his rose cracks right into Beryl’s body. The energy of his love for her causes Beryl’s body to deteriorate further, and she phases through the floor to beg for help from Queen Metalia. Endymion would soon die in Sailor Moon’s arms.
When Beryl begs Metalia for the power to defeat the Princess, Metalia is able to break her seal at last, and fuses herself with Beryl. The result is a green-skinned green-haired Beryl emerging from a purple flower, and she is about the size of a skyscraper. (As we see in the Silver Millennium flashback, an unsealed Metalia is roughly that size as well. We also see that in the SilMill battle Beryl’s hair seems to stand on end when she’s using a great amount of power.) Because it is a merging of both Queens’ powers, at that moment, she is by far the most powerful being on Earth. Even with the power of the Silver Crystal, Princess Serenity could not have defeated her on her own. Super Beryl also tries to break Serenity’s spirit by telling her that the world is not worth believing in. But, because Serenity combines her power with that of her friends, the pink energy ball defeats Queen Metalia and Queen Beryl at the same time, and seals them for good. The energy then grants Usagi’s dying wish to return to a normal life, and reincarnates her and her friends.