It is known that Maleficent hated Sleeping Beauty and got her to prick her finger on a spinning wheel and supposedly die. However, the spinning wheel had Maleficent's sleeping curse in it, and only true love's kiss could break the curse.
She first appears in her home, the Forbidden Fortress, when she is visited by Regina. They talk for a bit, and Regina bluntly ignores having polite conversation, demanding that she gets the Dark Curse. Maleficent, who had traded her sleeping curse for the Dark Curse with Regina long ago, refuses. This starts a small fight, which ends when Maleficent wastes her power on saving her beloved pet unicorn, and Regina traps Maleficent. The Dark Curse is hidden in the glass orb on Maleficent's staff, and Regina smashes it, obtaining the Dark Curse. Maleficent warns her that enacting the curse will leave an empty void in her heart, but Regina takes this all in stride and leaves.
S1, E2: "The Thing You Love Most"