The Grey Knights are the cream of the crop. The greatest of the great who bear weapons and armour unrivalled by any other chapter. They are the most elite human warriors unknown to man. There is a God of Chaos whose servants find their prowess an irresistible challenge and one of these daemonic servants intends to show his master that he deserves his bloody lord's recognition.
Grey Knights vs. Khornate Daemons because, honestly, Genestealers wouldn't stand a chance.