Warhammer 40k - Grey Knights Short Story - Crucible by John French

At the heart of a rift in space and time awaits a monster from the darkest imaginings of man, a daemonic being of immense power and evil. A lone Grey Knight, Brother Istafel, is sent on a desperate and suicidal mission to banish this dark entity. But as he approaches the Crucible rift, he realises that all is not as it seems... Why was he sent on this mission? What is the purpose? And do his masters expect him to survive?