According to Star vs. the Forces of Evil: The Magic Book of Spells, Eclipsa was born to Solaria Butterfly, but she lost her mother at a young age and inherited the throne. What is revealed whenever she is mentioned portrays her as the "black sheep" of the Butterfly family, with a dark and/or less-than-respectable reputation.
In the episode "Into the Wand", Star Butterfly travels inside her magic wand and learns about some of her royal ancestors via the tapestries in what she calls the "Grandma Room" of Butterfly Castle. According to a passage inscribed on her tapestry's pedestal, Eclipsa was wed to the Mewman king Shastacan, but she took a monster named Globgor as her lover and ran away with him.
During her life, Eclipsa developed a number of spells that were (or would later come to be) associated with dark magic, including the All-Seeing Eye. These spells were recorded in the pages of the wand's Magic Instruction Book, and her entire chapter was deemed "forbidden" and sealed behind a skull-shaped lock (which could only be opened by Glossaryck) to prevent others from reading them. She is also the only queen who never questioned Glossaryck's teaching methods.
According to Baby in the episode of the same name, Eclipsa had a very high affinity for magic compared to other queens, rivaled only by Star herself. It is also presumed from a remark by King Butterfly that Eclipsa harbored sympathy for Mewni's monsters, a sentiment shared by very few of her fellow Mewmans and royals.
In "Crystal Clear", Eclipsa is seen among the various interdimensional creatures that Rhombulus has imprisoned in crystal.
In Star vs. the Forces of Evil: The Battle for Mewni, Eclipsa appears in Queen Moon's flashback. Rhombulus briefly unfreezes part of her body so Moon can make a deal with her in order to defeat Toffee. Upon being unfrozen, Eclipsa asks Moon for a chocolate bar from the nearby vending machine, having gone for several centuries without any food, and she sympathizes with Moon, since they both lost their mothers at a young age. When Moon asks Eclipsa to teach her a spell that can kill an immortal, Eclipsa agrees in exchange for her eventual freedom, and they form a magical contract. As soon as they strike the deal, however, Rhombulus immediately freezes Eclipsa again. In the present, following Toffee's defeat and a brief visit from Moon, part of Eclipsa's crystal prison cracks, hinting at an eventual escape.
In "Stranger Danger", Eclipsa meets Star Butterfly after escaping from her crystal prison, and the two strike up a friendship. However, once her escape is discovered, the Magic High Commission put her in a dungeon. Although she is revealed to have committed several questionable acts in the past, including eloping with a monster, the Commission is unsure if she is truly evil, and Star convinces them to give Eclipsa a fair trial.
In "Lint Catcher", Eclipsa appears to be under some form of house arrest until her trial, chained by her ankle to a tree with dragon shackles as she feeds the local birds, and helps Star with her troubles concerning Marco. Eclipsa warns Star that, contrary to popular opinion, attending to the laundry is the most dangerous job in the castle, prompting Star to rescue Marco.
In "Sweet Dreams", Eclipsa saves Marco and Star from being lost in the void. When Marco reacts to her presence with hostility, she playfully mocks his karate stance. After Star ejects Marco from her room, she and Eclipsa talk, with Eclipsa revealing she managed to escape her confinement in her tower through the use of secret passages. Eclipsa encourages Star to embrace the dreams she's been having and see where they lead, telling her "all knowledge is good knowledge". Eclipsa then goes back to her tower through a passageway behind a picture on Star's bedroom wall.
In "Monster Bash", Eclipsa is revealed to be the mother of Meteora Butterfly, the true identity of Miss Heinous.
In "Total Eclipsa the Moon", Eclipsa helps Queen Moon solve a mystery involving the true identity of Eclipsa's daughter, with Moon reluctantly allowing Eclipsa to accompany her to the Bureaucracy of Magic's royal archives. Inside the archives, Eclipsa discovers that all records of her daughter Meteora were replaced by Festivia the Fun. After escaping detection by the Magic High Commission, Eclipsa is distraught at the erasure of her daughter's existence, but Moon believes that her daughter is still alive.
In "Butterfly Trap", Eclipsa's trial is held, which reveals that her daughter's erasure from the Mewni records was orchestrated by the Magic High Commission on orders from her ex-husband King Shastacan. Despite being found guilty of "crimes against Mewmanity", Star nonetheless sets her free.
In "Tough Love", Eclipsa works together with Moon to find Meteora as she wreaks destruction across Mewni. Upon reuniting with her daughter, Eclipsa tries to reason with her using tough love. When that fails, Moon attempts to capture and incapacitate Meteora by force, but Eclipsa intervenes, resulting in an attack that leaves Moon in a vulnerable state and forcing her to escape through a portal.
In "Divide", Eclipsa informs Star of everything that happened during the events of "Tough Love", and Star confines her to her chambers to prevent her from intervening during the upcoming battle against Meteora. In "Conquer", Eclipsa escapes her confinement and is forced to once again take up the royal magic wand to defeat Meteora once and for all. After Meteora is reverted to a baby, Eclipsa takes her and leaves Mewni to be with her husband Globgor again.
In Season 4, Eclipsa becomes the new queen of Mewni after Star relinquishes the throne to her. As queen, she forbids the wrongful imprisonment of monsters.
More coming soon...
When introduced in "Moon the Undaunted", Eclipsa is portrayed as calm, approachable, and sympathetic to young Moon's loss of her mother, despite her reputation as a queen of evil. Her demeanor toward Moon, however, provides little indication towards her true nature and motives, and she was initially hesitant to teach Moon her darkest spell. In "Stranger Danger", she bears no ill will toward her jailers, despite her long time in crystal captivity, and she displays a carefree, fun-loving attitude similar to Star's own. As of "Sweet Dreams," she maintains her calm demeanor and is more than willing to help Star, from warning her about the dangers of the castle laundry room to rescuing her from being trapped in a different dimension. She also has a playful side to her, casually telling Star to "call [her] a villainess" for her perceived crimes, then later responding to Marco's hostility with light-hearted teasing when the two of them first meet.
Unlike most magic users in the show, Eclipsa takes a more academic approach to magic, exploring "dark magics" for the sake of knowledge rather than malice. She notably encourages Star to ride out her unconscious Mewberty transformations to discover what they mean, and states her opinion is that "all knowledge is good knowledge".
Even after three centuries, Eclipsa has no regrets about abandoning her own kingdom and lawful husband, King Shastacan, for a monster. She cares deeply for her monster husband Globgor and is determined to discover the whereabouts of their daughter, Meteora. When she finally does locate Meteora, she attempts to convince her to halt her rampage, and prevents Moon from killing her. But when it becomes clear that Meteora cannot be reasoned with, Eclipsa, with a heavy heart, attempts to do the job herself. She is initially heartbroken when she appears to succeed, then overjoyed to learn that her attack only regressed Meteora back to her infant self.
She has a fondness for chocolate bars and roses, consuming a whole bucket of Snookers bars over the course of an afternoon.
Role in the series
Eclipsa would reprise her role as rightful leader of Mewni, and thanks to the actions of Coffee, the Lodgers would have sparing adventures in Mewni. It's revealed in the SAFA Chronicles that Mewmans are not native to the United Universes as their home dimension was destroyed by a Xexaxez, a Lovecraftian monster charged with eliminating dimensions that are too powerful and dangerous to the rest of the multiverse. Eclipsa, among other queens of Mewni, were also ruined by Prime Major Admiral Asper Swipe, a Mewman warlord and general of the Mewman Royal Guard with a magically extensive longevity who slain the original monster king of Mewni, originally called M'onstrarus, during Mewnipendence Day. Because of his vow against monsters because of the Mewmans' goal of finding and keeping their new home, Asper ratted out Eclipsa's relationship with Globgor, getting her crystallized and Meteora sent away. Once Eclipsa returned and was back in charge, she confronted Asper for ratting her out as well as his other crimes against the Butterfly family and they fight to stop Asper from planning to overthrow Butterfly Castle and waging a genocidal war against monsters. The battle ends with Asper almost winning before deciding that the fight wouldn't have beneficial changes, fleeing the dimension to restart his plan for conquest. Eclipsa would continue leading Mewni to this day.