From The Battle of Sancre Tor
Leaving a weak force in the lowlands to draw out the defenders, General Talos approached the citadel of Sancre Tor from the rear, descending the supposedly unscalable heights behind the citadel, and sneaking into the supposedly magically concealed entrance to the inner citadel. This remarkable feat is attributed to the agency of a single unnamed traitor, by tradition a Breton turncoat sorcerer, who revealed both the existence of an obscure mountain trail down the heights behind the citadel and the secret of the citadel entrance concealed beneath its illusory lake surface.
From The Rise of Tiber Septim
Accounts of Cuhlecains assassination differ greatly. The Pocket Guide to the Empire, 1st Edition offers the more commonly believe version. It states that a Breton nightblade murdered Cuhelcain before making an attempt on the life of the great general himself. The nightblade, it is said, managed to slice the general's throat.
Combined with The Arcturian Heresy, which plainly states that Hjalti killed Cuhlecain, I think it paints a very interesting portrait of Imperator-that-was.