Lion El'Jonson. Master of the First

Each primarch is a unique creation, born from the Emperor's genetic material and embodying different aspects of His personality. They all possess limitless potential, but only one has the right to be called the First. Lion El'Jonson embodies all the... best reasons the primarchs were conceived, and his legion is renowned for its endurance, honor, and unparalleled martial prowess. They are the loyal instrument of the Master of Mankind, His unwavering punishing hand. They are the Dark Angels.
While the Emperor gathers His mightiest offspring to assault Ullanor Prime, the master of the First leads his legion to the farthest reaches of the known Galaxy to quell a single rebellious world. Is this the indulgence of his infamously notorious pride? Or does the accursed graveyard of fallen empires, named the Wraith Stars, conceal something that Lion is not ready to reveal even to his own sons?