Pot of Greed
The Necklace is first, but only because he needed it and only because the Eye has consequences in its removal. After all, he takes his new duty as guardian of the Millennium Items very seriously. He has to, as Shadi is no longer there, and sealing the Items away failed along with the Ceremonial Duel. But above all, Atem is a strategist.
As they say, forewarned is forearmed.
With the Necklace carefully hidden under the collar of his jacket, and the power of both it and the Puzzle (which is his, always his and it will never be passed on) at his disposal, protecting the Items has never been easier.
Conversely, finding the Items' next bearers has never been more difficult. One would think, he muses, that the Necklace would make locating those destined to own the Items simple. Years pass however, and no person he sees, either with his eyes or with the magic contained in the Necklace, is the correct one for any of the six Items he has to give. Those years become decades, and as the decades inch into centuries, Atem decides to try another way. Surely with the powers of the Ring he can find the souls burdened with the destiny of owning the Items.
But even with the Ring no such person is found. It is, oddly enough, as if they are shrouded in shadow. Still, it is his duty and so he is determined. The Ring rests always with him, just under the Puzzle, in its never-ending quest to lead him to those souls.
Not long after he begins to wear the Ring someone takes notice. It's no surprise, really, considering the amount of gold he wears at that point. The Penalty Game he forces the would-be thief to play makes Atem realize that there is another duty he has, one that has been left untended for far too long. He picks up the Scales and the feather of Ma'at and weighs the hearts of those that would dare to try and trespass upon the souls of others.
Almost bizarrely, he feels guilty for it, although it takes him some time to discover why. Joey, Tristan, and even Seto; all of them had once been bullies or bad people, but they had been given chances for redemption. All of them had, in one way or another, redeemed themselves.
So it is that he takes up the Key, to view the soul room and decide whether or not the transgressor is truly beyond help. It clinks against the Ring and Puzzle, where it rests between them on a sturdy cord; within easy reach should he need it to deal with any crooks.
Centuries later, Atem is tired of hunting. Perhaps he has missed a bearer by being in the wrong place? Fate should have prevented it, but he ignores the quiet whisper and takes up the Rod. Never actively or completely controlling anyone, he uses it to seed the population with watchers on the lookout for those he needs to find.
It is only after he finally raises the final Item, the Eye, to his own socket with the flimsy excuse that surely he won't miss the next bearer with the power to read minds that he realizes he's been playing an intricate game; one that has spanned for just over a millennium and one that he has lost. But it's far, far too late.
The shadows croon in triumphant pleasure as, through all seven unique gateways to the Realm, they flood their avatar with silent power and promise. This world is theirs, and as they seal the single door with the ability to lock them away forever, crimson eyes gleam.
Long live the King.