Sack of Winterfell

The Sack of Winterfell is an event in the War of the Five Kings. It happens offscreen in the episode "Valar Morghulis".

Prelude
After news of the Fall of Winterfell to a band of ironborn led by Theon Greyjoy and the death of Ser Rodrik Cassel reach King Robb Stark, his vassal Roose Bolton, Lord of the Dreadfort offers to send word to his bastard son to gather a force to retake Winterfell. King Robb agrees but demands Theon be kept alive so he can execute him personally.

Some time later, after Lord Bolton reassures him that his son is only a day's ride from Winterfell, Robb orders him to send word to him that any ironborn who surrenders will be granted mercy, with the exception of Theon. Robb hopes to turn Theon's garrison against him and spare his people and brothers from any retaliation.

The Siege
After the Northern force reaches Winterfell, a horn is sounded constantly as means of demoralizing Theon and his garrison. Maester Luwin, bound to serve whoever rules Winterfell, suggests Theon to escape through one of the hidden passageways and flee to the Wall and take the black. Theon rejects Luwin's counsel and opts for a different plan.

Theon gathers his men in Winterfell's main yard and delivers a speech to them, in which he exhorts them to resist the Northmen and, even though they will all die, reach glory, ending with a frenzied cry of the prayer to the Drowned God: "What is dead may never die". Theon's men respond in various degrees of emotion. After the cry is over, Dagmer Clefjtaw knocks Theon unconscious and tells the garrison that its time to go home.

As Theon's men place a hood over his head, Luwin comes out into the yard asking to know what's happening. The sole response Dagmer gives him is a spear thrust to the stomach.

After this events, Winterfell is put to the torch and several, if not all, its inhabitants to the sword by an unrevealed party. Luwin manages to get himself inside the godswood.

Aftermath
Bran and Rickon Stark emerge from the catacombs of Winterfell, where they had hidden with their protectors Osha and Hodor, to find Winterfell covered in smoke and dying flames and its people dead. They also find Summer and Shaggydog on the gates of the godswood and enter it, only to find an injured Luwin beside the weirwood heart tree. Luwin urges Bran and Rickon to go north to the Wall so their half-brother Jon can keep them safe. As they set to leave Luwin tells Osha to protect the two boys and then asks her to mercy kill him. Afterward, the four leave the home of House Stark.

In the books
In the A Song of Ice and Fire books, the Sack of Winterfell takes place in a different manner. The Northern forces are rallied by Ser Rodrik Cassel, while the ironborn refuse to yield, as Theon takes the people of Winterfell hostage and threatens to hang them one by one if the Northern host doesn't disperse. The fate of of the siege is decided when a host of men gathered by Theon's sworn man "Reek" arrives. By the end of the siege, Winterfell is also put to the torch.