Story ~~
[episodes 1-5 only]
Instead of a Witherstorm, it's a Wither Possession. The Possession slowly moves person to person depending on who it kills, in a cloud of shadow. The command block is in the body of the person it possesses, and transfers as it moves.
These few people need to survive and protect all that remains of the world.


Better explanation ~~
If you get possessed by it and try to kill someone, you will die and the person you tried to kill will become possessed.
If you're the last person alive, the possession will live on in your body until it figures out how to get into other worlds, and the first person who gets possessed will come back alive after the possession gets "killed".


Jaythecat and Undeadwolf only.