The Last Dark — Stephen R. Donaldson

Compelled step by step to actions whose consequences they could neither see nor prevent, Thomas Covenant and Linden Avery have fought for what they love in the magical reality known only as "the Land." Now they face their final crisis. Reunited after their separate struggles, they discover in each other their true power--and yet they cannot imagine how to stop the Worm of the World's End from unmaking Time. Nevertheless they must resist the ruin of all things, giving their last strength in the service of the world's continuance.