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Having destroyed the outer elfgate, Arthas and his army pressed on into the Inner Kingdom, killing all in their path. Sylvanas ordered her forces to retreat over the Elrendar River, waiting alone to confront Arthas personally. She swore that while Arthas had gotten through the first gate, he would never breach the second, since the inner gate to Silvermoon could only be opened by a special key, which she would make sure he will never possessed it. Arthas watched her retreat, sneering that she was wasting her time trying to "outrun the inevitable". Sylvanas commented she was not running and remarked Arthas had clearly never fought elves before, upon which she used a magic arrow to destroy the bridge, the only crossing point over the Elrendar, thus delaying the Scourge's advance.
Furious, Arthas ordered his soldiers to find another way across the river. The undead fell upon a small village trapped on their side of the river, slaughtering the helpless villagers, destroying the buildings and raising their own base upon the ruins. Arthas's forces were bolstered by two new arrivals: greedy goblins willing to offer their zeppelins to the death knight, and a new form of solder; the spider-like Crypt fiends. These arachnid warriors quickly proved their worth when a force of dragonhawk riders attacked, shooting their webs over the airborne riders, bringing them down and leaving them at the mercy of the undead.
Having assembled a substantial force, Arthas used the zepplins to ferry his army to the nearest elven settlement to the east, which the Scourge destroyed swiftly. In the aftermath of the slaughter, Arthas found in the ruins of a temple a strange crystal. The shade of Kel'Thuzad told him the crystal was part of the Key of the Three Moons, the key required to open the inner elfgate. Knowing now what he needed to do, Arthas quickly reassembled his army and fell upon the next elven settlement, this time to the north. This village offered little more resistance than the first; it was quickly overrun and Arthas claimed the second crystal from the ruins of the temple there. Finally, the Scourge army fell upon the final elven village to the west. This one proved more of a challenge, for Sylvanas and her army were positioned there, and they fought like ones possessed to defend it. Though the efforts of the high elves delayed the undead for a time, ultimately, the Scourge would not be denied, and the settlement was taken. Sylvanas and her surviving forces retreated back to outposts they held in the forests while the Scourge ravaged the fallen settlement, and Arthas claimed his prize from the ruins of the third temple. With all three crystals in his possession, the Key of the Three Moons was complete and the way onward was now open to the Scourge.
Arthas retreated back to his base, where he began to gather his forces for a final assault on the inner elfgate. In a final defiant gesture, Sylvanas obtained some zepplins of her own, as well as more dragonhawk riders, and led them in desperate hit-and-run attacks on the Scourge, hoping to prevent their advance, but ultimately, once a suitable force had been raised, Arthas departed with his army and fell upon the defences of the inner elfgate without mercy. The elven defenders bravely fought to the end, but they were too few and too weakened to put up much of a defence, and were killed down to the last elf. With the defences gone, Arthas used the Key of the Three Moons to blast open the elfgate; the way to Silvermoon was now open to the dead. Sylvanas ordered her remaining forces to retreat to their holdings on the city's outskirts, wondering if there was any way to drive the undead back, while Arthas, growing ever more vexed by the Ranger-General's defiance, vowed to deal with her before he marched on the elves' capital.