Long before the demigods agreed with the gods that they would claim every child at thirteen, a group of children were running around with a satyr, trying to get to Camp Half-Blood. They were all quite lost and they were all trying to get back, bringing a few more as they went. They kept getting more lost, and finally, once they got there, one of them who they were sure was a demigod couldn't get in. The Mist didn't fool them, but they couldn't reach the barrier, they were stuck outside.
The child wanted revenge for separating him with his sister and friends, and he wanted it badly. Extremely badly.
