Ultraman for All: I Hold the Dark Circle

One day, when a universe was covered by darkness and all life in it was in danger, everyone on the earth was called to go to that universe and become Ultraman to save the world. Gais originally thought so too, but after looking at the dark ring in his hand and his strongest light system... Well, is there a possibility? Do I want to become light? This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.