So one day, a young boy found this weird tool in the middle of some old ruins and when he swung that tool it rips open a tear in spacetime, showing a different dimension. He would then enter in and explore it. He could do this over and over again every time, opening different dimensions. The dimensions vary from empty desert places to huge towns filled with weird people, massive ocean buy you can somehow breath underwater, to a dimension where mankind is about to get extinct and so much more.
This is an RPG so the boy will do "quest" in these different dimensions. Basically he'll figure out stuff, help the people there and get rewarded with items from that certain dimension. Later the collection will grow, and this items will be useful. For example certain items are needed for dimensions that you haven't visited yet or vise-versa and it keeps progressing.
Now, as he keeps traveling through dimensions. He get glimpses of this dark creature, not really sure what it is since it was just a glimpse but the further he goes, the more dimension he travels the more of these shows up. Until at one point after he keeps getting clues here and there from doing quests.. He realized..
"Everytime I rip a tear in space time.. They went out.. "
Every time he opens a portal to another dimension. A creature from the void, the gap between the dimensions, escapes. These are creatures of nothingness, they were born out of nothing, lived in the middle of nowhere between dimensions and everywhere they go they create nothingness, and by this time everything was too late. There were already too many of them set loose by the boy and they started creating nothingness here and there. Things start to disappear, people start to disappear. Everything, one by one starts to disappear and here comes the grand quest. The boy has to not only save the world, but save hundreds of thousands of worlds. He need to get rid of these creatures and save all the different dimensions.
Using an old ancient temple that is known to make portals between dimension, the way to restore it and to make it work is to use the items you have, including the tool that you found in the beginning. Opening the grand portal will suck the void creatures.