The Evil Queen had had enough. Her mother was trying to trick her into marrying. She'd even gone so far as to fake the Lion tattoo, the one on the arm of her true love. Regina had thought she'd walked away from that chance when she was younger, when Tinkerbell took her to the tavern. So she'd given Cora a chance. She had been foolish to do so.
The Lion tattoo had been placed there by magic; Cora's magic, of course. The man brought before her was not her true love. Anger boiled through Regina's veins, and her need to enact revenge upon her mother for her misleading grew to a dangerous level. And she did something that would affect her forever.
Part of the reason that Cora wanted Regina to marry was to produce an heir to her throne. Someone to continue the legacy. Cora had always wanted to be royalty, so maintaining that status mattered greatly to her. So what better way to spite her mother than to ensure she would never have an heir?
As Regina's assistant, you were asked to fetch a potion for her; she didn't elaborate on what it was for, but you obeyed dutifully. Once she had it, the Queen poured it into a wine glass. You couldn't help but wonder just what it was for, but you held your tongue on the matter.
When Cora arrived, she and Regina disputed; the Evil Queen was angry, of course. Cora attempted to argue that she really had thought the man was a good fit for her, but Regina wouldn't hear it. If Cora thought she wasn't going to drink the potion, wasn't strong enough to do this, she would prove her wrong.
Regina drank the potion from the glass in one gulp, then put it down on the table beside her. Then she doubled over, her expression twisting with pain. Alarmed, you rushed to her side, trying to aid her as Cora chastised her.
"Go back to Wonderland, Mother!"
Regina eventually growled through her pain, narrowing her eyes. Her tone so full on anger and hurt towards her mother, alongside the physical pain that the potion brought to ensure that she would never be able to have a child.