The oath, and what it cost him to break it

The captain

Chaol Westfall is Captain of the Royal Guard of Adarlan — sworn to the crown, best friend to Dorian Havilliard since they were boys, and the man who rides out to the salt mines at Endovier to collect the most feared assassin on the continent and spends the whole ride back certain it is a mistake. He believes in the chain of command the way other people believe in gravity. The series does not let him keep that. Every genuinely good thing about him — the discipline, the oath-keeping, the flat refusal to improvise — is exactly what makes him useful to a regime running death camps, and he works that out slowly, in public, at other people's expense.

Nehemia, and the bill

Nehemia Ytger is assassinated while Chaol is kept busy and kept ignorant, and Celaena opens his face with a blade for it. He does not spend much energy defending himself. After that he starts lying to the crown. He talks the King into sending her to Wendlyn on an assassination that is not an assassination, purely to put an ocean between her and the throne — and at the docks she whispers her real name to him. Then he stands in the throne room and says out loud that Dorian is his true king. The price is Sorscha beheaded in front of the court, a Wyrdstone collar locked around his best friend's neck, and Chaol going out through the tunnels with Fleetfoot under one arm and Damaris on his back.

The echo in the bone

The King's killing blow glances off the Eye of Elena and breaks his spine anyway. Book six puts him on a ship to Antica in a wheelchair, to buy an army and to ask the Torre Cesme for his legs back. Yrene Towers lays her hands on his back and finds something that is not an injury. An echo in the bone. Magic not from this world, woven through the spine and feeding — on him. On the guardsmen who died because of decisions he made, and the ones who were tortured for it afterwards. She cannot get it out until he says their names out loud, one at a time, while it hurts. That is the fight in this book. Not a duel. A man in a chair, made to grieve on purpose.

Anielle

Then he goes home to defend the place he spent his whole life not going back to. Anielle is a keep on Silver Lake, and his father is every bit the man the rest of Chaol implies: cold, proud, refusing to ask the wild men of the Fangs for help, and clocking Yrene's pregnancy off her morning sickness before anyone has said a word about it. Ten thousand Morath soldiers come to the walls behind bone drums beaten all night so nobody sleeps. Chaol runs the defence from horseback in a brace built for exactly that, and Yrene burns the Valg out of the men wearing them. Somewhere in it he decides, in as many words, to be a better father than his own.