Clearly, events have been shaping up for a final battle involving the entire magical world. All is revealed. Voldemort has entire armies behind him, and nothing can stand in their path. The situation is hopeless but the remaining Good wizards, of course, continue to fight anyway. The magical nukes come out and vast tracts of real estate get turned into scorched wasteland.
Eventually, all forces converge on Hogwarts, and we have the climax with Harry, Ron, and the rest of the Phoenix gang battling in front of the doorway to Slytherin House, the only part of the school that is still standing.
Voldemort arrives and with a gesture, ends the battle. He announces his new order and the New God that will rule forever more, who has been waiting inside Slytherin House all this time for his servants to report that all is ready. Voldemort sends Snape and Lucius Malfoy inside with the good news.
Malfoy emerges, babbling incoherently and then collapses to the ground, stone dead. Something is not quite right. Snape comes out moments later, announces all is ready. Voldemort, momentarily confused, recovers himself and decides Harry et al should be brought inside to witness his final victory.
Once inside, we see the new God of Evil for the first time. He immediately seizes Voldemort and drags him off to Hell. The curtain is pulled back and there's Dumbledore, miraculously resurrected/healed/rejeuvenated. Standing next to him is the Petronus stag, now revealed in His full glory as the architect of all things. We find out that Snape had repented and now, because of his vital role in tricking Voldemort and bringing about the final victory, he is redeemed.
Harry & friends suddenly realize they're not actually inside Slytherin House at all, but are rather in an entirely New World, standing in a broad, green meadow, with the sun brightly shining. Looking back through the doorway/gateway and surveying the devastation, it is now clear to everyone that the familar world of magic is too far gone to ever recover. Dumbledore remarks that all things must end.
In the distance a trumpet sounds. From all the corners of the earth, the surviving humans, elves, gnomes, etc... come forward to pass through the doorway to approach the Petronus and be judged.
Meanwhile back in the Muggle world, it turns out that on the way to school for year seven back at the beginning of the book, the Hogwarts train had actually derailed a few blocks north of Kings Cross, killing everyone on board and, coincidentally, smashing up the entire city block where the Ministry of Magic was. Entire Weasley family, everyone associated with the Ministry of Magic, you name it, they're all dead.
But this is all for the best, since everyone (*) is now in the New World, a world that turns out to be bigger and more wonderful than anyone could possibly imagine. This world also happens to be -- wait for it -- the one, primal, True World of which every other world is but a pale reflection.
The book ends with Harry & Ron & friends embarking on a grand new adventure about which we can't possibly write anything because it's just so indescribably wonderful. And they all lived happily ever after. Forever and ever. Amen.
(*) Everyone, that is, except for Hermione, who didn't believe.