If you have an online subscription with any of the other consoles, you're looking at a similar setup with a Nintendo Switch Online subscription. Firstly, you need it to play multiplayer games online. So Splatoon 3 shootouts, Mario Kart 8: Deluxe races, Diablo III raids, and the like will need an Online sub from now on. However, if a game is classed as free-to-play on the Nintendo store, you won't need a sub to play online - yes that includes Fortnite!