Tears of Blood

As war engulfs the Perdus Reach sector, a strange destiny entwines Master Kalidian Axryus of the Dark Angels Space Marines and the eldar of Craftworld Kaelor. As they clash in battle, the seers of Kaelor realise that only the Dark Angels stand between them and horrific demise at the hands of servants of the Dark Gods. Read it because
The Dark Angels Space Marines wage war on the eldar of Craftworld Kaelor, while the eldar try to stave off a far greater threat. From Tales from the Dark Millenium.