I need to preface my story with an explanation, hopefully, to fend off suggestions of "copycat" or plagiarism. My childhood story so closely resembles a recent posting, called "The Tree" that some may doubt that mine is an original story. I assure you that it is.
Others may not think of it is as a supernatural experience and I must admit that, as a young lad, I did not either. Only years later did I think that what we said at that time may have invoked something best not trifled with. On to the occurrence then.
My parents and many of my aunts and uncles immigrated to Canada from the Netherlands. Most of them settled in various areas of Southern Ontario. I was born in Canada as were most of my cousins and because my mother and my aunts and uncles were close, we made the treks to visit them quite a few times a year. We loved visiting our cousins and in the summer we would get to stay for a week at a time at different cousin's places and they in turn would stay at our house with us.
On one such occasion my older brother and I were staying for a week at my cousin "John's" country house, near Galt (now Cambridge), on 10 acres. We were maybe 12 and 13 years old, having great fun together. One of the games evolved into pretending that John's younger brother, Mark, was made invisible by us. Mark was around 8 or 9 years old and was enjoying this too.
We carried this game on too long and eventually Mark was no longer having fun and was becoming frustrated. He would say something and the three of us would say, "Did you hear something?" Or he would touch us and we would say, "I think I felt something but I am not sure what, but where is Mark"? Well, to prevent tears we decided to bring Mark back.
The three of us hatched our method and decided to use an "incantation" to bring Mark back into our dimension. We would say it in unison and then, POOF, we would all see Mark again. We decided to do this in the tree fort and invited the invisible Mark to join the three of us up there.
The tree fort was not a conventional box way up in a tree. It was a large platform made of two sheets of plywood with plenty of joists to support the floor. This was anchored at about eight feet up in a sprawling tree that supported the joists on its branches very sturdily. My uncle, who built the fort, was a skilled forestry worker and also a good carpenter.
It was now time for the incantation. The three of us said, in unison, "Evil spirits hear my plea, bring Mark back to me". Did it bring Mark back? Well, sort of. The game ended abruptly in a very unexpected way.
As we said the last syllable of the "incantation", the whole tree fort lurched and fell out of the tree! It did not tip and dump us out. The whole eight by eight feet of the fort just lurched sideways and down and flopped on the ground. The fort remained intact (I said it was sturdy) and there were no broken branches in the tree.
We were scattered on the fort's floor and other than a huge surprise and a few bumps and bruises no one was really hurt. Mark, by the way, was now very visible. We had no explanation for what happened and the whole "evil spirit" thing meant nothing to us, so we did not even think to mention it to my uncle when we told him that the fort fell out of the tree. We just figured it was weird. I can't remember what my uncle thought about the whole thing.
My brother and cousins (both ministers by the way) and I still talk about this when we get together. We all remember it pretty much the same way. Although we can't figure out how the fort could have fallen out of the tree "naturally" we do allow for that to be possible. In retrospect, however, we all agree that the "evil spirit" part of the incantation was a very poor idea.
Loved your story very strange that the fort fell down by its self, I think you are right that an evil spirit did it.