Why does Corlath want to take Harry and how does he do it?

Corlath explains that the girl is to be a political prisoner, to be respected by all, including him. Three days later, Corlath and two of his men sneak onto the Residency lands. Corlath easily walks onto the property, walks through the walls of the manor, and finds Harry, asleep. Feeling a moment of pity, he gently covers her with a cloak and says a spell to make her continue to sleep.