Okinodoku desu ga, Bouken no Sho wa Maou no Mono ni Narimashita.

This manga tells a story about new Demon Lord Vellmort XIII. Day after day he suffers from boredom, because all Underworld is already conquered by his predecessors, nobody (except maid Sarah) dares to oppose him and he is already sick of luxury food and jollifications with concubines. But some fateful day he has found a solution and together with Sarah has gone to conquer the Ground, populated by humans. Will he succeed in his plan or will he die by the hands of justice?