- "Maester Wolkan, send ravens to all the Northern Houses: Roose Bolton is dead, poisoned by our enemies."
- ―Ramsay Bolton after murdering his father.
The Assassination at Winterfell is an event during the War of the Five Kings, in which Roose Bolton, the Lord of Winterfell and Warden of the North, is betrayed and murdered by his own son, Ramsay Bolton.
- Roose Bolton: "Walda and I have some good news as well... since we're all together."
- Walda Bolton: "We're going to have a baby."
- — Roose and Walda announce their coming child.[src]
After having defeated the forces of Stannis Baratheon, the Boltons celebrate a hollow victory, as Sansa Stark escapes during the chaos of the battle with help from Theon Greyjoy. With the birth of Roose Bolton's child looming, Ramsay's position as heir becomes more uncertain, especially when Roose subtly hints that he will disinherit Ramsay in favor of the baby if Sansa cannot be located.
- "You will always be my firstborn."
- ―Roose Bolton's final words before he is killed by Ramsay.
During a meeting with Lord Harald Karstark about future plans for the North, Ramsay suggests they storm Castle Black as Sansa has probably fled there to find safety with her brother, Jon Snow. Roose immediately shoots down the proposal, saying that killing the Lord Commander of an ancient and politically neutral order like the Night's Watch will rally the North against them, chastising Ramsay for being a wild dog. They are then informed by the Boltons' Maester that Walda Bolton has given birth to a boy. Ramsay proceeds to congratulate Roose and hug him, while discreetly feeling for chain mail on his back. When he finds none, Ramsay stabs Roose in the chest. Despite the fact that Roose is Harald's liege lord and Roose avenged his father Rickard Karstark by personally killing Robb Stark, Harald doesn't intervene. Mortally injured, Roose collapses to the floor and dies, ironically in the exact same manner as when he killed Robb. Ramsay then instructs the Maester to send ravens out with the message that Roose was poisoned by his enemies. When the Maester hesitates, Harald reminds him that Ramsay is his Lord now. Ramsay is quick to solidify his position as Lord Bolton, luring his stepmother and newborn brother to the kennels, where he feeds them to the hounds.
With the deaths of Roose and the baby, Ramsay becomes the only living member of House Bolton and the new Lord of Winterfell and Warden of the North.
In the books
As of the most recent novel in the A Song of Ice and Fire series, Roose Bolton and the pregnant Walda are still alive.
While the Bolton storyline in Season 6 of the TV series surpassed the novels, it nonetheless seems very probable that this is going to happen in the next novel, The Winds of Winter (and isn't just an invention of the TV series). The most recent novel strongly implied that Ramsay might kill Walda's baby and possibly Roose himself, much for the same reasons as in the TV show: Ramsay has proven to be short-sighted and impulsive, Roose has lost faith in him, and Roose might make any child he has with Walda his heir instead (Roose outright expresses his fear that Ramsay will try to kill any children he has with Walda).
The Bastard Letter that Jon receives near the end of the fifth novel (before he is attacked by his own men) is signed "Ramsay Bolton, Trueborn Lord of Winterfell". Although the contents of the letter seem to be as least partially false, and it is doubtful that Ramsay wrote it - the fact that he signs as "Lord of Winterfell" may imply that he has already killed Roose.
In the "Inside the Episode" featurette for "Home", David Benioff directly points out that Roose's murder by Ramsay, unlike the poorly-received Coup in Dorne, wasn't a spur-of-the-moment decision but part of a growing tension ever since Walda announced she was pregnant in Season 5's "Kill the Boy". Benioff further explained how this wasn't meant to be an unexpected or gratuitous "surprise" by pointing out how Lord Karstark doesn't even flinch when Ramsay stabs Roose in front of him: it had become obvious to characters around the Boltons that inevitably, either Ramsay would kill Roose and the rest of his family, or Roose would disinherit and/or kill Ramsay.