Night of the Wolf
Occurring shortly after the rediscovery of Angron, the Space Wolves Primarch Leman Russ was supposedly dispatched by the Emperor himself to confront Angron on Malkoya. There, Russ demanded that Angron cease the wholesale massacre of civilians and defeated human enemies and halt the practice of installing the mind-altering Butcher's Nails in his own warriors or face censor on Terra. Angron scoffed at the notion and expressed doubt that the Emperor had truly sent him. He then went on to openly insult the Emperor as a slave-driver and even jokingly insinuated he might take the Imperial throne for himself. Horrified by this "heresy", Russ and Angron came to blows as their respective legions battled around them. Angron shattered Russ' Frost Blade Krakenmaw and Leman Russ destroyed Angrons Chainaxe Widowmaker, after a brief but intense duel Angron finally defeated the Primarch in single combat. The boasting Angron was soon silenced when Leman Russ revealed his true strategy, in the process of the duel Angron had allowed himself to be surrounded by Space Wolves forces, Russ had held back deliberatly in a desperate attempt to help his brother come to sense and beat the effects of his Butchers Nails. Though he had apparently lost the duel, Russ could have ordered his forces to kill Angron with just a single word but chose to withdraw instead. Despite this Angron still saw the battle as a victory, the World Eaters inflicted a greater number of casualties than they endured, and he saw it as a humiliation for the Wolves.
In truth, many such as Lorgar had seen the incident as an embarrassment for the World Eaters, displaying their lack of discipline and Angron's reckless tactics. The fight had revealed Angrons rage and arrogance as a great weakness.