Merrigan wasn't really an evil queen. Her royal siblings referred to her as the brat, and she lived down to it.
However, she was married to an evil king. When he lost a fight with a magic apple tree and got himself killed, she did whatever she could to hold onto the kingdom. Including lying to a powerful seer.
Merrigan was cursed: turned into an old woman, she was doomed to wander until she learned to care about others. With the aid of a magic book named Bib, she encountered a mermaid trying to regain her tail, a soldier with a magic tinderbox, a merchant obsessed with finding magical cloth, a warehouse full of orphans, and a princess allergic to peas.
Forbidden to retrace her steps, she moved from kingdom to kingdom, and totally by accident helped others find their happily-ever-after.