From the Steam store page:
Tired of invading dungeons? It's time you built your own!

Do you feel you are in a slump? Monotonously disembowelling evil guys in endless quests for gold and a slightly better pair of boots? You are starting to wonder... are heroics really your calling? Welcome to the War for the Overworld!

War for the Overworld is a Dungeon Management Game that employs your favourite aspects of the RTS and God Game genres. You play an all-powerful, malevolent Underlord, long banished to the Aether. With unrivalled dark power and an insatiable desire for bloody conquest, you will arise once more, and reclaim your realm!

In your absence, the Underworld lost its way. It became feeble and afraid of the Empire of the Overworld, which grew strong and boldly comfortable in its quiet retirement from evil-slaying. Carve out your dungeon from the soil and stones of the Underworld, and build dozens of unique rooms to attract a myriad of creatures to your halls to work and fight for you.

In addition to your faithful followers, you have destructive spells and powerful rituals to cast upon any unsuspecting goodly Hero (or rival Underlord) who dares to breach your unhallowed halls. With your unmatched dark powers and mighty army of bloodthirsty minions, you are ready for your War for the Overworld!