The Plutonia Experiment

After Hell's failed invasion of Earth, the UAC is rebuilt under new leadership and sets to work on the Quantum Accelerator — a device capable of force-closing dimensional invasion Gates. Early tests go brilliantly. Then the creatures from Outside take notice. A ring of seven Gates tears open simultaneously; six are closed before the research complex is overrun, everyone is killed or turned, and the seventh Gate remains open. Guarding it is the Gatekeeper — an ancient, enormously powerful demon capable of controlling Gates. A lone marine, on leave nearby and closest to the site, decides not to wait for reinforcements. He goes in alone.