The Ascent of the Lion, sometimes called the War of the Stag and Lion, refers to the war of 500AC in which House Lannister seized the throne from House Baratheon.
Prelude
Long before the Council on Bloodstone brought active talk of rebellion, many lords and Lords Paramount had grown dissatisfied with the rule of current king Harys Baratheon. Harys was preoccupied with feasts and tourneys over properly governing the realm (which had just emerged from a long and harsh winter). The last straw came with his appointment of his childhood friend, Alester Targaryen, as his Hand. Many lords thought the young Hand was far too incompetent for the important position and that the King had openly disrespected them once too many.
This growing discontent fed the machinations of Varyo Velaryon, a scion of Driftmark and Spymaster for the Golden Company (a company of sellswords invested by a loyalty to the Blackfyre line), who sought to place the Blackfyre claimant Aerion on the Iron Throne. He called many lords in secret to the Council on Bloodstone wherein he persuaded many to overthrow the Baratheons and place Aerion on the Throne. Most notable of the lords to support him was Loren Lannister, Lord of the Westerlands. Lord Gylen Hightower, who had long shared his father's ambition and belief that the Hightowers should replace the Tyrells as Lords of the Reach, saw the opportunity to gain his power and declared for Aerion as well.
While plans were made for rebellion,several steps were immediately taken to increase Aerion's legitimacy and secure support in secret among the houses of Westeros. Aerion was brought to Casterly Rock and wed to Lord Loren's daughter Ashara while naiming Loren as his Hand, securing the necessary forces to march on the capital. At the same time Loren's son and heir Damon was hastily wed to Aeslyn Targaryen, the head of the exiled royal house. Damon led the main Lannister force to King's Landing under the assumption that Aerion would claim the throne. Meanwhile Lord Gylen set the initial war plan in motion while the Lannisters began their march.
Important Battles and Events
March on Oldtown
Working quickly to provide an opening, Lord Gylen kidnapped Baelor Tyrell's eldest daughter Maude from within the Red Keep itself and brought her to Oldtown. King Harys Baratheon, who had grown smitten with Maude and sought to make her his new queen, was told that Maude's disappearance was actually a blatant slight against him. Against the advice of his council, he rashly marshaled most of his loyal Crownlands and Stormlands levies to march on Oldtown and confront Lord Gylen, leaving King's Landing only weakly defended by gold cloaks.
Meeting in Oldtown
Joined by Lord Baelor's heir Troy Tyrell, King Harys was invited into the Hightower by Lord Gylen. During the heated meeting, an enraged Troy grabbed at Gylen and threw him against a wall, breaking guest right, making Lord Gylen furiously demand the King and Troy to leave the city. Harys ordered his Kingsguard knight Thaddius Lannister, son of Loren and brother of Damon, to return Troy and his siblings Benjen and Mellara to Highgarden.
Sack of King's Landing
While Harys and the Tyrell forces were occupied in the South, Varyo Velaryon, with help from Aemon Estermont's troops already inside the city, was able to wrestle control of the Lion's Gate and Mud Gate away from the city's Gold Cloaks and let Damon Lannister's army into the city. The subsequent sack saw the death of many lords (including the King's brother, Joseph Baratheon, personally slain by Damon), and the capture of many figures of note (including the King's son, Rickon Baratheon). The sack sent shockwaves across Westeros, and many noble lords where caught completely by suprise by the Lion's daring move. Several regions of Westeros, most notably The North, The Vale, and Dorne, sealed off their respective borders with their militaries following the sack, while King Harys was still completely unaware that his seat had been taken.
Following the Sack, Aerion Blackfyre was crowned that same day. All of a sudden, and to the surprise of Damon Lannister, acting Hand in his father's stead, Varyo Velaryon suddenly turned on the new king within minutes of his coronation and killed him in a fit of rage. As explained by Loren Lannister when he arrived, the entire plan to crown Aerion had been a ruse planned by him and Velaryon at Bloodstone, away from the other lords. Their true aim was to crown Damon himself, with his claim legitimized by his marriage to Aeslyn Targaryen. Despite his reservations on the matter, Damon took the throne.
Battle at Stonehelm
Ser Ulrich Dayne of the Kingsguard and his brother Martyn Dayne, recently betrothed to Princess Sarella Martell of Dorne, led a ragged army of Hedge Knights and levies from the Stormlands and Dorne against the forces of Damon Lannister, the stormlords declared for new Lord Paramount Orys Connington and the Golden Company led by Robert Manderly. Martyn was taken captive during the fighting and was used to force Ulrich to surrender, where after both brothers were taken as prisoners to the capital.
Following the Lannister victory at Stonehelm, Edmure Stark, Lord of Winterfell, had notions to march south and sit himself on the Iron Throne after crushing the Lannisters, having aligned with neither Harys Baratheon or Damon Lannister to this point. His brother Jojen, who was involved in a secret relationship with Thaddius Lannister, vehemently opposed his brother's plan. The resulting argument and fight led to Jojen killing Edmure in a duel. Now Lord of Winterfell and Lord Paramount of the North, Jojen declared for the Lannisters alongside the so far dormant House Arryn. Dorne itself continued to declare neutrality, with Princess Sarella, recently risen to Lady of Sunspear, even encouraging her lords to declare Dornish independence, though it is unknown where this line of thought led.
The battle at Stonehelm proved to be a major turning point in the war as the forces of House Stark and House Arryn declared for Damon Lannister immediately following the victory.
Crumbling Trust
Thaddius Lannister betrayed his King and took his charges hostage, forcing Benjen and Troy Tyrell to duel each other to the death. Troy beheaded Benjen, and Thaddius took Troy, Mellara, and Benjen's severed head back to King's Landing as prisoners.
Though Harys initially planned to take Casterly Rock as retribution, he later received a "present" by raven from Varyo Velaryon, containing his son Rickon's flayed skin. Enraged, Harys' immediately rushed to muster his forces along with the Tyrells to march on the capital.
Burning of the Riverlands
The sellsword company the Bright Banners, led by Lord Varyo Velaryon, swept through the Riverlands, burning holdfasts and flaying lords who refused to bend the knee to the new Lannister king. Lord Randyll Frey took three thousand men and besieged Harrenhal when Lord Emmon Baelish did not immediately declare for the Lannisters, though Frey himself was taken captive.
Sack of Highgarden
When he learned that King's Landing had been sacked, Harys left Oldtown for Highgarden to muster the Tyrell forces. Lord Gylen Hightower led his men, along with men from the Golden Company, to the Reach capital to sack it after the Tyrells led by Lord Baelor had left with the Baratheons, leaving the lands weakly defended. Manderly men, under the guise of alliance with the Tyrells, opened the gates for Lord Hightower's force, and the castle was sacked, looted, and burned. They nearly destroyed the castle completely, as well as the surrounding Tyrell lands.
Battle at the Kingswood
Several months after the Sack of Highgarden and after most of the lords of Westeros had declared for the usurper, King Harys' force marching up the Roseroad was the last opposition to Damon Lannister's full ascension. The forces of the Usurper, approximatly eighty thousand soldiers, were commanded by Jojen Stark and Thaddius Lannister, while sixty thousand loyalist forces were commanded by Harys Baratheon himself. Jaime Florent, Harys' last loyal Kingsguard was taken captive by Lord Ronnel Royce. Lord Baelor Tyrell was captured as well while King Harys Baratheon himself was slain by Damon Lannister, marking the end of the first era, and the first day of spring.
Burning of the Riverlands II
Lord Aeron Greyjoy led ironborn troops through the Riverlands in an attempt to flush the Lord Marq Baelish and his army from his castle of Harrenhal after it became clear he was opposed to the new king. Lord Baelish did not send troops to aid his vassals, but instead sent his sister Alia Baelish in secret to safety in Essos before fleeing himself to become a bandit. Following Harrenhal's capture and the last opposition in the Riverlands flushed out, Edwin Frey was raised to Lord Paramount replacing House Baelish, and all of Westeros declared for the Usurper.