Powerful magic. My sister is also capable of dreamwalking, but she rarely practices.

[What Luna means to say is that she is totally better at it. The best.]

I typically enter nightmares to guide ponies into confronting and overcoming their fears, but in this case they are caused by an outside source and one cannot simply overcome them through introspection. It has proven far more expedient to simply do away with them as quickly as possible, though I still do provide counsel where I think it needed.