The Librarian's Acolyte

Within the great brotherhoods of the Space Marine Chapters, there are those who stand apart. The Librarians – powerful battle-psykers who use their connection to the roiling currents of the warp to devastate their foes – are feared and misunderstood by those they call brother, but are loyal servants of the Emperor nonetheless. One such warrior, new to his powers, Brother Loraeus is given his first task as a member of the Ultramarines Librarius. But when disaster strikes and enemies attack, Loraeus must channel his power to defeat them and save his life – and perhaps his soul. Read it because
It's a small insight into the trials that would-be Librarians must face in order to be accepted into their Chapter's secretive brotherhood of psykers. Also, it has nurglings in it.