There is a human invading his sanctuary, and he is determined to get rid of her. Humans in the past have left of their own accord, but not without a threat from him. However, this one is proving difficult. But she’ll go. One way or another, he will make her.
The thing is demanding that Lark leave, but she has nowhere to go. No one who cares. So she's determined to stand her ground.
After all, what does she have to lose, besides her life?