Spellcross: The Last Battle

Colonel Alexander was cut off behind enemy lines with a few squads when the situation stabilized enough to rejoin the alliance. What he returns to is a war against something that does not follow conventional military logic: dimensional gate keepers have opened portals and sent armies of demonic creatures through them into the world. Alliance cities have been subjected to torture and sacrificial experiments on a systematic scale. The purpose, as intelligence eventually determines, is the activation of a sleeping titan buried within the Earth's core — a weapon larger than anything the alliance has fielded or faced.