Treason

"- What you suggest is treason. - Only if we lose."

- Ned Stark and Petyr Baelish

Treason is a serious crime against a sovereign or the realm or kingdom as a whole, including rebellion,  regicide, oathbreaking and espionage. A Queen consort committing infidelity against the King is also considered treason.

The punishment for treason is death, usually by public execution. If expedient, a traitor may instead be pardoned in exchange of hostages or by agreeing to take the black and join the Night's Watch.

Like oathbreaking, the nature of treason is not an absolute one and is subject to political machinations. It can often depend on whether said treason is successful. While a defeated rebel will be condemned and executed as a traitor, a victorious one will be in power and will merely receive the epithet usurper.

In the city of King's Landing, those accused of treason are confined to the Black Cells.

Major cases and accusations of treason

 * The lords Robert Baratheon, Eddard Stark, Jon Arryn, Hoster Tully and, later, Tywin Lannister, as well as their bannermen, households, and followers, commit treason against their King Aerys II Targaryen when they rebel against him. However, as they are victorious in their rebellion they are never judged as such. Robert Baratheon, however, becomes known as "The Usurper" by those who, secretly, remain loyal to House Targaryen.
 * Grand Maester Pycelle betrayed Aerys II when he counseled him to open the gates of King's Landing to Tywin Lannister when the latter presented himself before the city gates offering protection against the rebellion.
 * Jaime Lannister violated his Kingsguard oath and murdered his king, Aerys II, during the Sack of King's Landing. He was subsequently pardoned by King Robert.
 * Eddard Stark, acting in his authority as Hand of the King, decides to move against Prince Joffrey Baratheon and prevent his coronation by taking him prisoner, having discovered that Joffrey is a bastard with no right to the throne. Petyr Baelish reminds him that such action will be considered treason only if they lose. Baelish and Janos Slynt then betray Lord Stark, who is arrested and then executed for treason against Joffrey, accused of seeking to install himself on the Iron Throne.
 * The rebellion of the North and the Riverlands to secede from the authority of the Iron Throne is regarded as treason by those who remain loyal to King Joffrey.
 * Theon Greyjoy betrays Robb Stark and joins his father, Balon Greyjoy, in his conquest of the North.
 * Lord Rickard Karstark is executed for treason against King in the North Robb Stark for the murders of hostages Willem and Martyn Lannister.
 * Lords Walder Frey and Roose Bolton commit treason when they conspire with Tywin Lannister to eliminate Robb Stark and his family.
 * Tyrion Lannister is put on trial for the regicide of King Joffrey.