It was smart of Holly to inquire about the curse. Seneca wouldn't have much respect for her if she had blindly agreed to being her subject.

"It is an ancient Polynesian curse. People there believe in ghost sickness. What muggles didn't know is that this sickness was caused by someone, a wizard, who summoned this demon to possess the person. When someone has ghost sickness, they don't feel like themselves for they are possessed by a demon. I saw the effects on my maid -- she was hearing voices which were forcing her to do harm."

She omitted the part about the curse driving people to suicide.

"As for its uses, I suppose it can be used to defame an enemy."