World Prince Campaign
Initially, the World-Prince had refused to surrender before the Imperium and had stubbornly resisted the Imperial Fists led by Rogal Dorn. The Fists battled Hive by Hive and were making slow but clear progress until the Alpha Legion suddenly arrived. The Legion began operations on the world unannounced save stating that they were there as allies to the Fists.
Though no Alpha Legion forces could be found on the ground by the Fists, their mark was clear. Uprisings and coups erupted across the remaining Hives, throwing them into chaos. Finally the entire family of the World-Prince was massacred by native Humans assassin believed to be working for the Alpha Legion. Each assassin was someone close to their target. 401 members of the planets ruling nobility were dead, and in the aftermath the World-Prince became a broken man. He surrendered to Rogal Dorn personally, believing him responsible for what had befallen him.
In the aftermath of the campaign, Rogal Dorn finally confronted Alpharius in the throneroom of the World-Prince. Alpharius was accompanied by his lieutenants Ingo Pech and Kel Silonius, and attempted to hide his true identity. However Dorn guessed correctly and sent a blow at the Legionnaire on the Prince's throne, proving himself right when the Marine displayed Primarch-level reflexes to block the blow. Upon confirming the true Alpharius, Dorn scolded him for his deceitful way of waging war and demanded that all his operatives leave the world. Alpharius responded that the Emperor had created him to wage war this way, and the same was true with Dorn and every other Primarch for that matter. After Alpharius criticized the seeming ineffectiveness of the Imperial Fists method of war, Dorn proceeded to lay out the Alpha Legion's plan and poked holes in its effectiveness before leaving Alpharius.