Sam and Chatur both once again traveled to the Fysi castle to deliver Night Charm. Sam did expect that she would come after him. But he didn't expect that she would come the very day he was given the task to find her.
He must say he is lucky.
As for why the thunder prison was already prepared. It is because, he was sick of these people using that weird shadow token to escape every time, Sam was ready to finish them off.
He couldn't think of any methods instantly and decided to try his luck with the thunder prison and when he saw the results he was ecstatic.
But he didn't make this trip for her. He just thought that since the first attack failed, the second and third attacks wouldn't be far away. So, he just wanted to be prepared. He is just damn too lucky to have caught her.
He immediately decided to take her to Fysi Palace to finish the mission.
