Queen Metaria is a formless, malevolent energy being—basically the personified concept of evil itself. She's what happens when radioactivity in the sun takes conscious form, and she's the real power behind the Dark Kingdom, though she tends to work through others rather than directly.
The manga version has her orchestrating events during Prince Endymion and Princess Serenity's wedding, exploiting anyone who opposed the union. Beryl was her main target, a woman so jealous of Endymion that she became the perfect tool. After the conflict ended, Metaria got sealed away at the North Pole in suspended animation. Fast forward to the present day: Metaria manages to revive Beryl and sends her after the Silver Crystal as a key to her own freedom. When Endymion's reincarnated self gets captured, Metaria just possesses him outright, using his body as a puppet. With her host secured, she lets Beryl die in battle—no longer useful. It takes Sailor Moon ultimately destroying her to end the threat.
The anime plays things similarly, though here Metaria eventually grows strong enough to resurrect independently. When Beryl is dying, Metaria takes over her body and transforms into something godlike and towering.
Worth noting: the English dub calls her the Negaforce instead, treating the character more as a faceless dark lord or demon god of the Negaverse rather than the scheming mastermind Metaria represents in the original. The dub's version was never sealed away and seems content to stay shadowy while Beryl handles the dirty work.
(Her name's been romanized various ways—Metalia, Metallia—but Metaria is the author's preferred spelling in later manga editions.)
Content compiled by AnimeList.moe from publicly available sources.
