Many knew of the legend of Ahzek Ahriman, one of the most powerful psykers in the galaxy and ex captain of the XV Legion The Thousand Sons. Many knew of his exploits, how he traveled across both the webway and the warp and acquired fell weapons of sorcerous power.
Many knew of his failures, how he tried to save his legion and in doing so turned many of his own brothers into dust, how he was banished by his Primarch - his
father.
And now he toils in the webway, a euclidean dimesion which the Eldar use both as a form of transport and housing, in the search of the mysterious Black Library.
Ahzek Ahriman knew the truth. He knew that what he was doing would amount to nothing, even if he breached the formidable defenses of the Black Library he would have to fight every inch to get what he wanted. The Black Library housed the majority of the knowledge belonging to his home galaxy, he was not naive to think it housed every knowledge of the universe. The Clowns knew this as well but would never say that to anyone but themselves.
But it holded the knowledge necessary to revert the effects of that dammnable spell of his, and return his brothers to their flesh and sentience once more. When this goal of his was complete then he can die for all he cared, his task was done. But until then he will carry on no matter what.
Even if it took ten thousand years he would fight ten thousand more if his brothers would live again. But he was not blind, he planned again and again for centuries to get the Black Library and he was pushed back every time.
This was... the twelfth time he reached it again, it seems that he lost his perception of time again.
The Siege of The Black Library by Ahzek Ahriman seemed to have going just like the others, but there would be change this time. And it would favor Ahriman. The Forces of the Cherished Son for the first time in millennia breached the inner sections of the library. Ahriman did not wait he immediately searched for what he came for.
The Harlequins would eventually push back the forces of Ahriman but after the battle they discovered that a single book had been taken by Ahriman. The Cherished Son finally took what he wanted and he would never search for it again.
Ahzek Ahriman after millennia of warfare won.
+++++++++++++++++++++++++++++++
Fun Fact: Ahriman is actually right, while the Black Library houses the majority of the knowledge of the milk way galaxy it does not house all the knowledge of it or the warhammer 40k universe.