It starts by one person making a wish. Any wish.
The next poster will "grant" the wish (by simply typing "Granted" and will proceed to corupt the wish in some way. That person will then make a wish of their own and the process repeats itself.
Me: "I wish for a pony."
You: "Granted. But the pony is weak and can't support your fat !@#. You attempt to ride it and break its spine."
(Game brought to you by: Sunbreeze of Static Limit via Joeschmo of Executive Decision)
So I'll get us started with the first wish:
I wish for the Corrupted Egg of Millagazor