You are the dictatorial ruler of the Epsilon Galaxy, who has obtained complete and unchallenged authority. However, during a scientific experiment in hyperdrive your scientists have discovered four "buffer" dimensions, each containing a number of barren planets, that exist between the Epsilon Galaxy and other alien dimensions. You must battle against the four other aliens species for control of these buffer dimensions to ensure that your rule in Epsilon remains intact.
Overlord is a real-time strategy game where you must manage resources, both in military forces and economic and political stability throughout the galaxies. To make the new planets useful you can purchase terraforming equipment which can transform them into tropical gaia's which are ideal for agriculture, mineral-rich and heavily volcanic worlds, desert planets capable of producing vast amounts of energy, or ecologically balanced planets that can support large cities that will generate a high amount of tax revenue.
You can mine for resources and tax your citizens to raise money, construct buildings to feed people, enlist soldiers and equip them, buy battle cruisers or other craft to move from planet to planet, and so on. You can fight battles, colonize planets, and even spy on alien races. The end goal is to ensure that the other alien races do not gain the upper hand by conquering these four new dimensions before you can.
You cannot retreat from battle, as the only way to shut down the hyperspace link from their universe to yours is to vaporize your homeworld (which is where the hyperspace generator is located), which is politically (at the very least) not an option.
Blood will have to be shed for supremacy of the hyperspace planets. And hopefully not yours.