|Although this article is based on canonical information, the actual name of this subject is pure conjecture.
- "Casterly Rock is an impregnable fortress. But as a good friend of mine once said, 'Give me ten good men... and I'll impregnate the bitch!'"
- ―Tyrion Lannister to Daenerys Targaryen
The Fall of Casterly Rock was an engagement in Daenerys Targaryen's war for Westeros in which the Unsullied army of House Targaryen attacked Casterly Rock, the seat of House Lannister, who traditionally rule the Westerlands.
Tyrion, serving Daenerys as her Hand, develops a strategy to take Casterly Rock using the elite Unsullied army, commanded by Grey Worm, that Daenerys brought over from Essos, which will cut off the Lannisters from their stronghold and will also deal a heavy blow against their morale, given that Casterly Rock is their traditional seat of power. Tyrion expects a large force, at least 10,000 well-trained soldiers, to be holding Casterly Rock because of this; the castle has never fallen before.
Transported by a large portion of the Targaryen fleet, the Unsullied army of House Targaryen lands at Casterly Rock, where they attempt to climb over the battlements with ladders. The Lannister garrison has anticipated the attack, and fiercely defends the walls firing arrows and throwing large stones. The Unsullied initially sustain modest casualties and cannot seem to breach the walls. However, Grey Worm leads a small force of Unsullied through the sewers beneath the Rock provided by Tyrion, who installed a route that he would use to sneak in with prostitutes. Once inside the castle, Grey Worm and his force open the gates, allowing the rest of the Unsullied army to pour in. Following fierce fighting, the Lannister men are ultimately overpowered.
After the fighting has finished and the Lannister garrison has been destroyed, Grey Worm quickly deduces that the larger part of the Lannister army is mysteriously absent from the assault. He looks over the battlements to see Euron and the Iron Fleet destroying their ships. Out of anger, Grey Worm grabs a dying Lannister soldier and asks where the rest of the Lannisters are. As it turns out, Jaime has marched the Lannister host to take Highgarden. After taking the castle, he comments to Olenna that the Unsullied will be unable to hold Casterly Rock as he has emptied all the larders and will starve unless they leave, and with their fleet burned by Euron they will have no choice but to march cross country through a hostile Westeros.
Ultimately, however, Jaime's strategy, while ingenious, backfires: Euron's presence on the western coast of the continent allows Daenerys to land her Dothraki khalasar on the eastern coast without interference, and her victory at the Goldroad shatters the Lannister/Tarly force, thus allowing the Unsullied to march across Westeros unhindered.
Some time later, Grey Worm appears in front of King's Landing with the surviving Unsullied, having abandoned Casterly Rock.
In the books
In the A Song of Ice and Fire novels, an equivalent battle has not occurred. The TV show has surpassed the timeline of the books, so it is unclear if an equivalent battle will occur in a future book or not.