The official discussion topic on The Return of Sithis

Some more lore.

The 100 Day War: The name given to the hundred day invasion of the Falmer. On the first of Frostfall, in the 960th year of the 4th era, the former Snow Elves rekindled a millennia old war. Falmer poured out from underground caves all over Skyrim, thousands at a time, completely catching the Nine Holds by surprise. Before any real resistance could be formed, the Falmer captured various large forts around Skyrim, and began mounting a massive campaign to conquer Skyrim. Because of how widespread the Falmer were, and how many there were, it was almost impossible for the Holds to combine forces, and this left each Hold to practically fend for themselves. The first couple of months were bloody and horrible, and for a while, it looked like the Nords would lose. Then, everything changed, when General Darus Stormcaller, leader of Whiterun's forces, led a massive assault to take the two Falmer Forts, Fort Greymoor and Fort Sungard. He was successful, but in the end, he gave his life doing so. The remaining Falmer in the Whiterun area fled, freeing the mighty army of an enraged Taryn Stormcaller to aid the other holds. Enraged and grieving over the loss of her husband, Jarl Taryn destroyed Falmer by the thousands, calling forth the wrath of the gods themselves. It is said she was so terrifying, her men compared her to a Daedric Prince, in hushed tones. The Companions finally broke their long standing law not to intervene in the wars of Skyrim, and every Companion down to the last man followed their Harbinger into battle. The College of Winterhold even joined the war, and it was said, you could see the Archmage wading through Falmer corpses. Even the Thalmor set aside old grievances, at least for a while, and joined the fight against the Falmer.

In the fourth month of bitter fighting and desperate struggles, the last of the Falmer strongholds was retaken, and the war ended. Most of the Falmer were killed, and the few remaining fled back underground. Monuments were erected in every hold, honoring the fallen, and a new order of Sentinels was formed, to stand guard over Skyrim's darkest reaches, and patrol it's depths.