an unexpected and unpredicalment happen for one unfortunately soul-

A human was drag into an unknown portal that just appear from a lake she is passing by and some sort of shadow just took the trainer into the portal who just minding their own businesses-

...Giratina who was bored watching it realm in silent observing from the pond on one of the distorted platform just so happen to see someone passing by- decided to cause some mischief, maybe get some entertainment from scaring a mortal by dragging them into it dimension in attempt to get a look of the reaction by the 'guest', it decided to pull this trainer into it world.