My brother has severe night terrors?!

For a few years now, my nine year old brother has been having night terrors. About a week ago he had two in two nights. They usually last about a half hour, and during them he'll scream, cry, run around the house, and shout things at my parents, thinking that it's not them. Sometimes he'll feel their faces and realize it's them and say something like "I love you". I'm always really scared for him and just stay in bed. I always wonder; what can be THAT bad? What can be so bad that he'll do all of this? Any ideas about what he's seeing?