There's a Fourth Deathly Hallow in Harry Potter, and It's Not What You'd Expect
What do you mean by fourth, you ask? The Elder Wand, the Resurrection Stone, and the Cloak of Invisibility are canon-proof, there is nothing else!
Wrong! One fan theory suggests that there might be a fourth Deathly Hallow.
Let's say the three established items each have a special power: the wand gives you the ability to "wield death as a weapon," the stone can help you "reverse death," and the cloak is the tool you use to "escape from death."
Are these abilities enough, or could there be more that the Master of Death would possibly want?
The thing that comes to mind is the ability to "see death" – remind you of anything? Moody's Magical Eye!
We know that it can see through anything (even through the Cloak of Invisibility, which is supposed to fool Death himself!).
If it can do that, that means that the eye has powers at least equal to, if not greater than, one of the Deathly Hallows.
We know nothing of the origins of Moody's Eye, and Alastor was a paranoid wizard who was considered the best Auror of his time.
Maybe he had a tool that helped him gain that title? The tool would have to be rare and extremely powerful.
Also, we know that the eye was made specifically for Moody since it kept malfunctioning when Barty Crouch Junior wore it.
Assuming that the eye is a Deathly Hallow as well, another question arises: is Moody the fourth Peverell brother (very old or practically immortal), Death himself in disguise out for some human fun (being an immortal being can get lonely sometimes, right?), or, what's more likely, a descendant of the fourth brother?
Given that the eye seems tailor-made for Moody (or at least for a rightful owner), Alastor could be a great-great-great-grandkid of the fourth Peverell brother.
However, all the other Deathly Hallows could have been won or stolen, and they would have worked for you regardless.
Is it just that the eye's powers work differently, or does something not add up? So many questions, so few answers…
Still, it is a fun theory to think about, and it opens up so many possibilities for fanfiction!