Halo by William C. Dietz

After fleeing the fall of a human colony, a battered UNSC cruiser stumbles upon a vast, mysterious ring installation, where a Spartan super-soldier must lead marines, scientists and an AI companion in fighting both a fanatical alien alliance and a virulent parasitic species that converts the living into monsters; as the infection spreads and tensions rise, the survivors scramble to understand the ring’s purpose and stop its catastrophic power from being unleashed across the galaxy.

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