This story is going to be very short and basically just a written down idea.
Supposedly Albus gave Mad-Eye Moody his eye. However I believe that since The eye could see through Harry's cloak from death, yet death himself couldn't find it, that the eye was death's eye. There was a fourth Peverall brother. The four are Antioch, Cadmus, Ignotus, and Oswalt. This is also why the three others take the form of an eye. The eye, the stone, the wand, the cloak. Everybody forgot about the eye and the fourth (youngest) child because he hidden himself away from history. The eye could see everything. It could see death everywhere. This is what mad moody even more paranoid than most ex aurors.