'Beast' is so harsh, I prefer Maleficent.
Maleficent is a recurring character on Once Upon a Time. She was an evil witch who lived in the fairytale land, and who presumably had a grudge against Aurora's family. At some point, Maleficent was granted access to the Dark Curse by one of her only friends, Queen Regina, traded for her famed sleeping curse. When said sleeping curse failed to have the results Regina expected, she demanded to be given the Dark Curse back, and a battle ensued between the two witches, one which Regina won. When the curse was set in motion, Maleficent became trapped in Storybrooke, Maine, in a form rather different to that of the curse's other victims: her dragon form.
Before the Curse
Love is weakness, Maleficent...
Maleficent sends a message. ("Forget Me Not")
When Will Scarlet joins the Merry Men, he convinces Robin Hood and the rest of the band of thieves to steal from Maleficent's Forbidden Fortress, as rumors say there is a mass amount of gold stored there. Robin, at first, is reluctant, as he knows that Maleficent is a powerful sorceress, but Will has it on good authority that the witch shall be vacant from the palace, and Robin eventually concedes. The Merry Men later break into Maleficent's castle and steal the gold, however, whilst there, Will also steals a Looking Glass. When the men are later celebrating around a fire, Maleficent uses her magic to project her voice and make strong winds overcome them; she tells them to keep the gold, if they must, as it is useless, but to return the other treasure they stole, for although its power is alluring, it shall only bring them misery. Will, despite having stolen this other treasure, doesn't fess up, and later uses the Looking Glass to transport himself and his true love Anastasia to Wonderland. ("Forget Me Not")
Maleficent asks Rumple to join her. ("Heroes and Villains")
When Belle, working as Rumplestiltskin's maid, is out doing the laundry, she becomes distracted by a small dalmatian which she follows as it runs away. She eventually loses before someone grabs her from behind, kidnapping her. When looking for his maid, Rumple receives a message from a sand dollar which depicts a captive Belle; she tells him that he has to meet with her kidnappers and give them his magic gauntlet in exchange for her life. He later arrives at the a cliff with the magic gauntlet that points to a person's greatest weakness, and he meets who he expects to meet: Maleficent. Thinking it's just the two of them, he begins to strangle her with magic, but he stops when a pair of tentacles begin choking Belle. The sea witch Ursula reveals that she is there too, and then so does Cruella De Vil, much to Rumple's shock. He is hesitant about handing the gauntlet over, but when Ursula begins strangling Belle with her tentacles, he quickly tosses it over to Cruella, who explains that they shall use it to point to their enemies' weaknesses and then they shall be the victors. Rumple leaves with Belle whilst the self-proclaimed Queens of Darkness leave with the gauntlet, only to have Rumple corner them later and steal it back with ease, for one must have a death wish to demand a ransom from the Dark One. The Queens lament their loss, and Cruella points out that the game is rigged and that the villains never win. They suggest that Rumple join then so that they can change the game together, but he refuses, saying that he wins and he wins alone. ("Heroes and Villains")
Charming threatens Maleficent. ("A Land Without Magic")
Some time after, Prince Charming makes a deal with Rumplestiltskin in order to retrieve his mother's ring. He must conceal a bottle of true love encased in a protective egg inside a great beast. Maleficent enjoys a quiet moment in her home when Prince Charming suddenly appears with his sword to Maleficent's throat. Maleficent thrusts Charming back with a magical blast and reveals that she is “the beast.” She then magically blows out all of the candles in the room. Shrouded in darkness, Charming tells the witch to show herself, and slowly she reveals herself as a monstrous, fire-breathing dragon. A battle ensues, with Prince Charming finally forcing the egg down Maleficent's throat. His work done, Charming leaps through a stained glass window, diving into the waters far below. ("A Land Without Magic")
Maleficent gets a visit from Regina. ("The Thing You Love Most")
Frustrated, Queen Regina goes to Maleficent to get the Dark Curse back. Maleficent says they made a deal and the curse belongs to her now. She keeps the curse held in the orb of her staff, and when she refuses to willingly hand it over, Regina challenges her. They have a magic spar, and just as the Queen readies many enchanted weapons to throw at her pet unicorn, Maleficent jumps in front to intervene and is tied up with the metal of the chandelier, and is thrown against the wall. She believes she will be killed, but the Evil Queen states she would never do that since Maleficent is her only friend. Free to get the curse from the orb of Maleficent's staff, the Evil Queen is warned of the curse's dangers, and how it would leave a void in her, but she takes this all in stride and leaves. ("The Thing You Love Most")
After the Curse
Tell me, your Majesty, is our friend still in the basement?
Emma awakens the beast. ("A Land Without Magic")
In Storybrooke, Maleficent's dragon form is trapped deep below the town's library. In order to retrieve the same bottle of true love implanted inside of her by Prince Charming, Emma must face Maleficent. Quickly realizing she's not much good with a sword, she drops her father's weapon and draws her gun. She fires at the dragon, but the bullets seem unable to penetrate the beast's scaly hide. Emma runs, ducking behind a large pillar of rock. The dragon circles around, landing on a nearby outcropping to get a better angle of attack. But the outcropping crumbles and the dragon plummets into the darkness. Emma peers over the edge, but sees no sign of Maleficent. Suddenly, the dragon flies upward, the gust from its great wings knocking her back as it spots her. She fires her gun again, desperately, but all it seems to do is make the beast angrier. The dragon turns on her when she hurls the sword into its throat. The sword starts a chain reaction and an explosion engulfs the dragon, destroying it. As cinders blow around the cavern and smoke wafts amongst a giant pile of ash, Emma spots the golden egg containing the love potion, and gets her hands on it, at last. ("A Land Without Magic")
There's a powerful spell here that sustains her in whatever form she's in.
An undead Maleficent attacks Hook ("The Evil Queen")
When Regina has plans to enact the curse's fail-safe and wipe out Storybrooke along with its inhabitants, she invokes the help of Captain Hook who accompanies her down the library's elevator where they enter the caves beneath. Hook begins to wonder what Regina meant when she said the place is guarded, and Regina reveals that an old friend, meaning Maleficent, is doing so, explaining that a spell put in place around the caves sustains her in whatever state she's in. Hook then wonders why it's a two man job to receive the trigger, and Regina reveals that she needs him as a distraction, before bushing him off a cliff. Hook lands with his life as atoms begin to form together to create a skeletal creature and a discrepant nightgown that bellows spine-chilling shrieks: Maleficent. She attempts to attack Hook who tries to engage in small talk with the creature, not being sure how to fight back, as Regina retrieves her fail-safe trigger. Eventually, Hook is rescued by Greg and Tamara, however, it is unknown how they defeated Maleficent or what her current state is. ("The Evil Queen")
After the Second Curse
If you want to find the truth, drink up.
Emma remembers all, including the dragon she once slayed. ("New York City Serenade")
When Peter Pan arrives in Storybrooke and casts his own Dark Curse upon all of it's citizens, Emma Swan and Henry Mills leave the town for New York to avoid it's affects. They're given new memories by Regina before the Evil Queen undoes the curse with her powerful magic and sends everyone still in the town back to the Enchanted Forest to avoid Pan's new one. A year later, Emma is happily living in New York with Henry when Captain Hook, who managed to cross realms and track down Emma, approaches her with a potion that will help return her memories. Emma believes him to be crazy at first, but eventually she trusts him and drinks the potion. Her true memories are thus returned and she begins remembering her life, including the time she fought and killed Maleficent in her dragon form. ("New York City Serenade")