Let this scar signify the first blow against the mortal world. From this seal shall arise the doom of men, who, in their arrogance, sought to wield our fire as their own. Blindly they build their kingdoms upon stolen knowledge and conceit. Now they shall be consumed by the very flame they sought to control. Let the echoes of doom resound across this wretched world, that all who live may hear them and despair.
He came to destroy the world, and he nearly did. If not for the combined efforts of the orcs of the Horde and their allies under Warchief Thrall, the human refugees led by Jaina Proudmoore, and the night elves under Tyrande Whisperwind, Archimonde the Defiler would have walked up to Nordrassil, the World Tree that sits atop Mount Hyjal, and he would have consumed it utterly. With it, he would have gained all the power of the Well of Eternity that has seeped up the tree's massive roots over tens of thousands of years, making him quite possibly the most powerful entity Azeroth has ever seen, even including beings like Deathwing, the Old Gods, and potentially even Sargeras himself.
In the end, however, the mortal races banded together in a desperate alliance and held Archimonde and his Burning Legion away from the tree long enough for Malfurion Stormrage to play a final, desperate plan that Archimonde couldn't anticipate, because it involved self-sacrifice, the loss of the night elven immortality. For Archimonde, who had lived for more than 25,000 years constantly coveting power, the idea of giving it up was unthinkable. Combined with his ego and arrogance, he was effectively blind to the danger, and so he died.
We hope. Make no mistake -- Archimonde, of all the Burning Legion, came the closest to actually destroying Azeroth. Neither Sargeras nor Kil'jaeden ever walked Azeroth bodily. Archimonde did. Neither Sargeras nor Kil'jaeden ever set eyes on the Well of Eternity. Archimonde stood mere yards from it, drawing the power from Nordrassil. The Defiler very nearly did exactly that to all of Azeroth.