Lady, last winter while the rest of us stayed snug with the garrison at Gothregor […] Niall stowed away in a supply wagon and went south with the Host. He served as a messenger in the battle at the Cataracts. We don't know exactly what he saw there—he won't tell us—but it must have been awful. Now he's afraid that he'll fail Tentir because he can't stop the nightmares. He thinks only cowards have bad dreams.
– Mint, To Ride a Rathorn, "Chapter X: Battles Old and New", I
He served as a messenger at the Battle of the Cataracts. His experiences at the Cataracts give him screaming nightmares in the barracks, which cause Vant to give him punishment; eventually, Jame comes down from her attic to talk to him and hear his story. She expresses sympathy and shares her own story of nightmares about the Cataracts (she has repeated dreams involving Kin-Slayer and a changer who is originally Tori, but becomes Ganth and repeats what he said when he cast her out). The rest of her ten-command and the other listening cadets realize that Jame, Niall and Brier are all veterans of that battle, and that nightmares don't make Niall a coward; he just needs to learn to face them and not let them overcome him. The next morning, he looks happier, and Jame is relieved.
He is described as "thin and quiet" and "sober", seeming older than peers.
He got a white stone for field surgery after Killy pinned a randon to the ground with a lance through the leg.
↑To Ride a Rathorn, "Chapter X: Battles Old and New"
↑To Ride a Rathorn, "Chapter X: Battles Old and New", I — "Thin and quiet by day, he looked gaunt and haunted by candle-light, and he was shivering."
↑Bound in Blood, "Chapter V: Fractures", II — "She and Brier, although only a few years older, seemed ancient by comparison, except perhaps for sober Niall, who had snuck off with the Host and seen more than he had bargained for in the red carnage of the Cataracts,"
↑Honor's Paradox, "Chapter XVI: Tests", II — "Niall also scored for field surgery after Killy pinned a randon to the ground with a lance through the leg, thus earning a black for himself. Jame suspected that Damson had rigged the accident since the plump cadet liked neither Killy nor that particular instructor. Moreover, she was fond of shy Niall, whose battlefield experience at the Cataracts was well known."