The thing that really makes my army "scary" to opponent is the three Venerable Furioso Death Company Dreadnoughts. These three models contain most of the killing potential in my army. As such, they are always a priority target for the enemy, and I try to shield them behind my tanks or terrain for as long as possible. Of course, they are all named and have a little back-story.
REDEMPTOR - Once a Chaplain, Brother Redepmtor led one of the largest Death Companies ever assembled in the Second Battle for Armageddon. This massive formation of blood-crazed Marines smashed into the Ork flank, disrupting their lines and delaying the Ork capture of Hades Hive. None of the Marines survived, except Redemptor...his broken body consumed by the Black Rage, he was entombed in a dreadnought sarcophagus so that he could continue to battle the enemies of humanity.
RESURGAM - One of the oldest Death Company Dreadnoughts, no elder or record identifies the true name of this battle-brother who has served his chapter for millennia. He is known only as Resurgam, meaning "I shall rise again". In countless battles, his dreadnought body has been all but annihilated, only to rise up once more, tearing his would-be slayers limb from limb.
ALLISON VALENTINE - Over 300 years ago, Allison Valentine was serving as a Sister of Battle to bring glory to the House of Valentine. But, in a combined action with the Blood Angels chapter, she was overcome with the Black Rage. The Blood Angels believed this to be a miracle, as no one outside their chapter and without their gene-seed had ever fallen to the visions of the Black Rage. They immediately encased the dying woman in a suit of dreadnought armor, so she might live on. The Ecclesiarchy and Allison's fellow Battle-sisters were dubious, suspecting the taint of Chaos rather than a miracle. It was this event that forced young Jonas Valentine into exile. Now, Borther and Sister fight alongside one another; she in the un-death of a dreadnought, he with the pronounced longevity of the Blood Angels. Continuing suspicion of Chaos taint is why Inquisition forces accompany the II MARS detachment, ever watchful of some sign that the Valentines are in fact agents of darkness.