The Silver Spike (Chronicles of The Black Company #3.5)

...embedded in the trunk of the scion of the godtree, it contains the essence of the maddest of the Ten Who Were Taken...The Dominator. Defeated by the Lady and cast from this world, all that was left of him was a foul trace of lingering evil. But the graveyard that was once the Barrowland contains more secrets than dead. All who would possess the power of the Dominator are drawn to the spike. A foolhardy band of thieves is the first to reach it, and a rapacious and malign spirit is unleashed on an unwary world. The forces gather, sides are drawn, and mortal men can only die as the Dark Lords battle for domination.