Trying to complete that PokéDex and not sure what that empty spot after Ninjask is supposed to be? It's...Shedinja! Shedinja is an extremely unusual Pokémon that cannot be caught in the wild or fought in Max Raid battles -- it's only obtained through a special evolutionary trick using Nincada and Ninjask.
While Pokémon Sword and Shield introduces several new evolutionary methods, fortunately, some other things just never change. Shedinja was introduced along with its family members Nincada and Ninjask in Pokémon Ruby and Sapphire, and its evolutionary method has remained consistent ever since.
How to obtain Shedinja
Shedinja is fairly easy to obtain. All you need is a Nincada, and a spare PokéBall. Have Nincada in your party, as well as one additional open slot (so, a total of five or fewer Pokémon in your party), and at least one PokéBall in your bag. Then, evolve Nincada into Ninjask by raising it to level 20. You'll find a Shedinja added to your party in addition to the Ninjask once it has evolved. The Shedinja will also be at level 20 and will know the same moves that Nincada knew when it evolved.
That's all it takes!
Can I fight Shedinja in Max Raid battles?
No. Shedinja is only obtainable through the method described above (or by trading with a friend who already has one).
Where to find Nincada to evolve into Shedinja
Nincada are relatively easy to find. They're only in Surprise Encounters (by running through tall grass and going toward rustling grass, rather than seeing them on the overworld), and can be found in any weather on Route 5 with a 5% chance of encountering one every time.
If those odds are too low, head to the Wild Area and look for them in Surprise Encounters at South Lake Miloch in Intense Sun or a Sandstorm, or West Lake Axewell during a Sandstorm. The chances are highest during Sandstorms in either location, at 40% and 30% respectively.
Why do I want Shedinja?
Unless you're making a very unique competitive battling team, you're probably only after Shedinja to finish your PokéDex. That said, Shedinja is a very unusual Pokémon that can have some utility in competitive teams built to support it.
What you'll probably notice first about Shedinja once you get it is that it has only 1HP -- which seems like it would render Shedinja a waste of time to battle with. However, it also has an ability called Wonder Guard, which causes it to only take damage from Super Effective attacks, weather, and condition effects (like Poison). Since it's a Bug/Ghost type, that means only Fire, Flying, Rock, Ghost, and Dark-type moves can damage it.
That's still quite a few kinds of attacks that can hit it, and with moves like Stealth Rock, Snow Warning, and Toxic so prevalent in competitive battling, it may be tough to make Shedinja a useful part of your team. That said, it can be worth trying alongside a held item like a Focus Sash and a set of powerful moves. If you use Shedinja, good luck!
Still confused about Shedinja? Let us know in the comments below, and we'll try to help you out!