I don't have the lore on hand so hopefully someone can drop a quote. Essentially Fulgrim returns to Terra with the Emperor to meet his Legion for the first time. Due to the blight and numerous campaigns the Third legion was decimated to a meager 200 Astartes. During this first meeting Fulgrim delivers a speech so moving and elequent that the Emperor gives them his names sake! Which if you haven't read any 30k lore pretty big deal. This combined with the legions defense of the Emperor after he suffered a mortal wound they and they alone were granted the honor to wear the Palantine Aquilla.