The Greater Good — Sandy Mitchell

When the world of Quadravidia comes under attack by the insidious tau, only one man can defeat the aliens and save the planet in the Emperor's name: the legendary Hero of the Imperium, Commissar Ciaphas Cain. When the aliens call for a ceasefire, Cain expects the worst, and his fears are answered in the form of the dread menace of the tyranids. As a hive fleet approaches Quadravidia, Cain must try to forge an alliance between the Imperium and the tau - but can he truly trust the inscrutable xenos?