If you fall on your sword you take the blame for a group action that might not have been your individual responsibility.
He doesn't think he's to blame – but he's falling on his sword anyway.
She's going to resign - she's falling on her sword even though it's not her fault.
He is the boss – it's only right he's falling on his sword this time.
Don't confuse it with
Another phrase which means to take the blame for something is to take the flack.
She's always taking the flack for things. Even when it's not her fault.
The honour of a Knighthood in the United Kingdom comes from the days of Medieval chivalry. The King or Queen touches the person who is going to be knighted on their shoulders with a sword.