There were so many of them. Knights surrounding you. Regina was trying to spark another fireball, but her minimal practise was showing, and the knights were getting closer. You knew what you had to do - it was just a matter of whether you could.
You had once been a powerful evil sorceress, seeking revenge upon King Stephan for the wrongs he had done to you in the past. Your name was known and feared by all - but your fire had been stolen. Ripping away your powers, and causing you to give up hope. That was, until Regina paid you a visit.
Regina somehow convinced you to follow her out to the remains of a scorched forest that you had burned while in your dragon form. One tree remained alight, so Regina had brought you to it to reclaim your lost fire so that you could use powerful magic once more. You'd been caught doing so by the knights that were surrounding you.
While you had been bitterly resigned to your fate as you were tied up, Regina wasn't - she managed to break out of her restraints, alerting the knights, and placing you in the predicament that you both found yourselves in. You knew what Regina was waiting for you to do - out with the weakened sorceress, and enter the dragon.