Bearing in mind that Eliezer consistently foreshadows important events, let's brainstorm what Harry might do to end the world.

First, ritual magic and Dark rituals have played a prominent role in the story. Dark rituals have been mentioned over and over, and it's been emphasized that they are dangerous and powerful.

A magic ritual, much like a magic potion, seems to achieve much more than a spell. In my opinion this is probably because of the same conservation law by which potionmaking uses a small amount of magic to unlock the power already in some sense inherent in the potion's ingredients. (Example: Lily Potter sacrifices her life and successfully provides Harry with lifelong magical protection, while not even Dumbledore is capable of casting such a spell even with the Elder Wand.)

In fact, I conjecture that Eliezer brought potions into the story the way he did as a way of making us aware of this magical conservation law. A potion, to my mind, is merely a certain class of magic ritual which produces a magically potent substance. I may be wrong, since we don't know that potionmaking obeys the same ceremonial formula as a ritual.

Here's Quirrell's description:

An ordinary Charm, Mr. Potter, can be cast merely by speaking certain words, making precise motions of the wand, expending some of your own strength. Even powerful spells may be invoked in this way, if the magic is efficient as well as efficacious. But with the greatest of magics, speech alone does not suffice to give them structure. You must perform specific actions, make significant choices. Nor is the temporary expenditure of your own strength sufficient to set them in motion; a ritual requires permanent sacrifice. The power of such a greater spell, compared to ordinary Charms, can be like day compared to night. But many rituals - indeed, most - happen to demand at least one sacrifice which might inspire squeamishness. And so the entire field of ritual magic, containing all the furthest and most interesting reaches of wizardry, is widely regarded as Dark.

So, I think Harry will either discover or, equally likely, invent a magic ritual which sacrifices something really ... squeamishness-inducing ... in exchange for root access to the Atlantean mainframe, or whatever.