Before the Series
Rowan mentions that Cairn was once part of his army while he was under Maeve's command and that no matter what he did (lock him up, flog him, discipline him or try show compassion towards him), Rowan couldn't break Cairn's sadistic personality. He therefore kicked Cairn out of his own army and dumped him onto Lorcan's lap, where he lasted only a month before he was transferred to an isolated legion. Rumor says that he did unspeakable things to both the soldiers and civilians he came into contact with.
Fenrys and Gavriel reveal to Rowan that Maeve has replaced him with Cairn. Rowan expresses horror upon receiving this information, recalling his failed attempts at ridding the warrior of his sadism. When Lorcan also failed the same objective, Cairn was sent to an isolated legion.
Cairn appears in person when Maeve confronts Aelin Galathynius on the beach along the Gulf of Oro. Under Maeve's command, Cairn brutally whips Aelin and even smirks at the tattoo on her back. Before departing with Maeve, he deliberately leaves Aelin's shirt behind in the pool of blood as a message to Rowan.
Cairn spends the next few months torturing Aelin, using every horrific method he can think of to break her, including breaking her bones, making her kneel in broken glass, and flaying her. When he attempts to burn her, Aelin fights back and manages to get free with Fenrys's help; while Fenrys attacks Cairn, Aelin runs out into the camp they're in and Rowan arrives at the same time. He makes sure Aelin is safe, then goes back for Cairn, who wets himself when he sees Rowan. The Prince skins him alive, then beheads him and burns the remains.
Personality and traits
Cairn is described as incredibly sadistic and cruel. He is also seen as uncontrollable in that respect, as both Rowan and Lorcan tried and failed to rid him of that brutal nature.
He is considered a coward when he does not have the obvious upper hand.
Cairn had brown hair and blue eyes. Elide Lochan notes he might have been handsome if not for the obvious sadistic cruelty radiating from him.
As a Fae, he has slightly pointed ears and sharp canines. His animal form is unknown.