- "During the Dance of Dragons, the Red Kraken picked the side that was fighting the Lannisters and fell upon the Westerlands while the Lannisters were off at war. Casterly Rock itself proved too strong once the lady of the castle barred its gates, but the ironborn burned the Lannister fleet and sacked Lannisport carrying off gold, grain and hundreds of women and girls including Lord Lannister's favorite mistress and all his bastards."
- ―Euron Greyjoy
The Sack of Lannisport was part of a campaign of the House Greyjoy against the Westerlands during the Dance of the Dragons. Although Dalton Greyjoy, the Red Kraken had passed his offensive as an action in favor of Rhaenyra Targaryen, this was in fact an excuse for the Ironborns to resume their Old Ways of raiding and enslavement on the coast of the Sunset Sea. The success of the Greyjoy offensive led them to maintain their attacks on Westeros until after the end of the Targaryen civil war.
Dalton Greyjoy, also known as the Red Kraken, tried to attack the Westerlands during the Dance of the Dragons. Although he didn't succeed in taking Casterly Rock, he did however burn the Lannister fleet and sack Lannisport by taking the gold, grain and women of Lannisport; including the mistress of Lord Lannister and her children.
In the books
In The World of Ice & Fire, Dalton Greyjoy was approached for allyship by both the Greens and the Blacks during the Dance of the Dragons. King Aegon II Targaryen offered Dalton the position of Master of Ships and admiral of the crown's armada, as Ser Tyland Lannister had recently stepped down from the latter position to become Master of Coin. Dalton was offered these positions on the condition that he bring the ships around Westeros and battle Lord Corlys Velaryon. At the suggestion of her uncle and husband Prince Daemon Targaryen, Queen Rhaenyra Targaryen appealed to Dalton's bloodlust and simply asked him to attack her enemies.
Dalton accepted Rhaenyra's offer and attacked the Westerlands, which were vulnerable as Lord Jason Mallister was fighting for the Greens in the Riverlands. Lord Jason's wife, Lady Johanna Lannister, barred the gates of Casterly Rock. Dalton burned House Lannister's fleet and sacked Lannisport, pillaging gold, grain and trade goods and taking hundreds of women and girls, including Lord Jason's favorite mistress and their bastard daughters.