This is similar to the story game in which each person adds a little bit to the story, but the twist is that each post has to start alternately with "Fortunately" and "Unfortunately".
Example:
If I started: "There once was a duck named Millard Mallard, who lived in the desert far, far away from the nearest duck pond."
The next person might write: "Fortunately, he was a wealthy duck, so he was able to buy a nice house with a big pool and a gorgeous pool boy."
And the following person might write: "Unfortunately, the pool boy's secret ambition was to be a French chef. His favorite dish? Duck a L'orange."
And so forth, alternating back and forth. Here goes:
There once was a troll who wanted to be a Knight of the Round Table.