One block as name say`s one block is a survival world with only 1 single block. its like sky block the block floats in the mid air and you get spawn on it. And you have to survive on it.
One block is one of the most popular mod of Minecraft game. in the mod you get spawn on a single block with nothing, you have to break the single block over and over to get new items.
One block have different stages like cave, snow, end, etc.
Each and every stage you get new types of blocks like first you get dirt than stone than iron and it skips going.
at the very stage you will get end stage and end portal you can use the portal to travel to end and kill the ender dragon.
Is One Block Free?
Yes one block mod is free for every on you have to download and import to your minecraft game than you can play one block map.