crossover fiction
placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story
en-GB/ˈkɹɒsəʊvə ˈfɪkʃn/
en-US/ˈkɹɔˌsoʊvɚ ˈfɪkʃən/
placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story