Haarlock's Past

Erasmus Haarlock - Erasmus Haarlock was a Rogue Trader and master of the vast power of the ancient and infamous Haarlock family on the world of Quaddis located in the Calixis Sector. The Rogue Trader House of Haarlock is an ancient one, far predating the Calixis Sector, which it helped to found. For thousands of standard years, the Haarlock Warrant of Trade gave the Haarlocks license to travel far beyond the Imperium and to bring death and war to anything they deemed a threat to Mankind’s place in the cosmos. Over their centuries of conquest, they put whole worlds to the sword and plundered the tombs of xenos races passed to dust. They acquired terrible secrets and amassed dark lore, weapons, and trinkets enough to found a dozen empires of their own and condemn them in the eyes of the Imperium a thousand times over if it were not for the legal protection afforded by the Warrant and the secrecy with which they guarded their affairs within the Imperium. Ever a fractious line, the Haarlocks were prone to betrayal and internecine warfare. Erasmus lost his beloved wife and only daughter to a vicious war between the members of the Haarlock line as they fought for the inheritance of the Haarlock Rogue Trader Warrant of Trade. There is one thing that Erasmus Haarlock desired after the death of his wife and daughter; to undo what happened, return them both to life, and return to the life that he led before they were slain. Haarlock was willing to pursue any means to this end. He later hunted down and killed every member of his family and their families, friends, allies, and associates. After slaughtering his blood kin, he unified the dark lore that his line had amassed over the millennia, and willed them to a single purpose, a terrifying project that spanned a dozen worlds and brought into play art and sciences forbidden since elder days. Then, at the height of his labours with all in preparation but for the final piece of the puzzle, Erasmus Haarlock vanished. The last of his line, centuries later, he is presumed deceased. The huge wealth of his house is locked in legal dispute in the Courts of Solomon on his former homeworld. Though some whisper that scions of the Haarlock line may still exist, they have proven reticent to come forward to claim the great prize of Haarlock's legacy.

From a Lore Expert on the matter

His house was greatly part of bringing Imperial Rule to the Calixis Sector, so officials were rather lenient in pursuing them for possession of forbidden technology and lore. And I mean not only xeno technology like having an eldar sword, or having a small moon where he built a bio dome so he could grow orcs. I'm talking of learning Xeno languages and not only dabbling in the Warp, but understanding and mastering it, so much he was able to bind demons to his will and force them to do his bidding. Erasmus Haarlock was a force to be reconed with, a man no living being could dare to stand against. A man where even immortal beings like demons chose to bow to his will rather than fight. (There are rumors that he is able to take the souls of those he kills, destroy those and take their power. Essentially this means he was able to effectively kill demons for good.)
He lost his wife and kid during the assassination attempt, and fueled by rage and a wish for vengence, he moved out to kill the rest of the Haarlock dynasty, to take all their knowledge and power and combine it, in an attempt to bring back his lost loved ones. With all his dynasty dead, him being the last survivor, he moved on to prepare a ritual. During that ritual, he disappeared, together with his ship and crew.
And there are rumors of time travel.
Long ago during the birth of the Calixis Sector, the Rogue Trader line of Haarlock brought with them across the stars a servant to protect their secrets. Ancient and terrible, it knew every price the Haarlocks paid for power; it was named the Widower and brought death to any who would try to learn its master’s secrets. The Widower was a protean terror, a thing of many shapes and endless masks. It was bound to the strange power of the Steel Clock at the heart of the Haarlock estate in the city of Xicarph and only obeyed the will of a scion of the Haarlock line. Prophecies whisper that it will reappear in the time when the sun above Xicarph turns black and the Light of the God-Emperor is lost. The Pilgrims of Hayte, a nihilistic warp cult, seek to claim Haarlock’s Legacy and believe that the Widower can reveal its secrets. So, the Pilgrims have come to Quaddis to claim the Widower through a diabolical plot. They have bought the services of the infamous Beast House slavers to bring them individuals who have been marked by warp spawned prophecies and amongst whom they hope to find one who has the blood of Haarlock and can command the Widower to reveal the Haarlocks’ secrets.