Spellcross: The Last Battle (1997)

The story line pits the alliance against the forces of darkness, the game begins with Alexander(you) a colonel in the alliance forces who has become trapped behind enemy lines with a few squads of men, fighting your way through the forces of darkness you meet with the rest of the alliance and are brought to your first mission. The computer plays against you as the gate keepers or under lords who have unleashed a terrible onslaught on the earth torturing and killing many of the population they have conquered in sacrificial magic experiments. The story unfolds that they are attempting to awaken a sleeping titan within the Earth's core.

The forces of darkness use portals and gates to different dimensions to bring hordes of demonic creatures into the world.