In the underhive depths of Rigo V, the Sorcerers Ramon and Phaelius of the Thousand Sons seek proscribed knowledge. They are hunted, these witches, by the Rune Priest Thorskir who has tracked them across the length and breadth of the galaxy. At last after an arduous search, Thorskir has found where his prey will be and means to end them. But the plans of those allied to the Great Architect of Fate are not so easy to unbind and a secret lurks beneath Rigo V, one that has been long in the devising, a twist of fate and a plan so foul it is worthy of Tzeentch itself.

Read it because
It's a dark and twisted tale playing on the enmity between the Space Wolves and Thousand Sons, and the weirdness of anything touched by Tzeentch.