This is just a small feature I've been thinking about for spiritomb for quite some time and decided to post it since spiritomb has been announced.
As you all know the premise of Spiritomb is that there are 108 souls trapped in Spiritomb odd keystone. I thought a really cool feature for spiritomb would be this;
Have the odd keystone as a drop from mega bosses much like the orbs for the legendary bird shrines. And just like those orbs you keep the odd keystone in your inventory and defeat 108 pokemon just like the 108 souls trapped in the keystone. Once the stone is full it becomes a place able block and you can place it to spawn spiritomb.
It's just an idea I had that I thought would be pretty cool and thank you for reading it!