This is the 16881st most frequent Polish word.
wymyślonym
"Imagined" or "made-up."
In this sentence, 'wymyślona' acts as an adjective derived from the verb 'wymyślić' (to invent/come up with), describing the noun 'historia' (story) as something invented or imagined.
To jest wymyślona przez niego historia.
This is a story imagined by him.
Here, 'wymyślonym' serves as an adjective modifying the noun 'świecie' (world), portraying the world as a product of imagination.
Opowiadał nam o wymyślonym świecie pełnym cudów.
He told us about an imagined world full of wonders.
In this instance, 'wymyślonymi' is an adjective agreeing in case, number, and gender with 'zasadami' (rules), and signifies that the rules were created or derived imaginatively.