Complex sentence: Like all of the four major Chaos gods, Nurgle is opposed by one of his fellows-in this case, Tzeentch, the Chaos god of change and sorcery.
Split into simpler sentences: Like all of the four major Chaos gods, Nurgle is opposed by one of his fellows. In Nurgle's case, his anthithesis is Tzeentch, the god of change and sorcery.