When I reincarnate, I'm a cube!?

After dying of exhaustion on Earth, I seem to have been reincarnated in another world, but...the landscape that unfolded there was like an old video game, a world where everything was a cube. Then the fairy Erin appeared and led me on a journey through this cubic world. This is a light FPS in quest format. The basic rule is that if you wipe out all the demons in the stage, the quest is complete. When you complete a quest, a new quest will be generated for you to challenge.