Bjorn: Lone Wolf (Black Library Advent Calendar 2013 #1)

As daemons swarm across the fields of Velbayne, bringing madness and Chaos to all that they touch, Leman Russ and the Rout stand arrayed against them. Packs fight back to back, brothers protecting one another except for one. The Lone Wolf, the warrior without a pack, stands alone. And somewhere on the battlefield is a foe against which he will have vengeance.
Read it because
It's a small but perfectly formed look at Bjorn of the Space Wolves in the later years of the Heresy, taking revenge against the beast that massacred his pack. Through the story, he reaches his defining moment and his purity of purpose is revealed.