Pokémon have always had various ways of evolving into new beasties, and Pokémon Sword and Shield are no different. One of these curious contenders is the tiny and wonderful Sinistea, which can be found primarily in Glimwood Tangle, just south of the town of Ballonlea. Pokémon don't really like showing their faces in this area, so your best bet to find one is just to rummage through the tall grass and running into exclamation marks.
Once you've nabbed a Sinistea, you're going to need to source a special item to make it evolve, namely the Cracked Pot. You can buy one from the market vendor in Stow-on-Side. He's the chap on the right that you can see in the image below, and his stall is to the left of the Pokémon Centre. His inventory cycles daily, so it's possible he won't have a Cracked Pot for you to buy, in which case you're going to need to keep coming back day after day. It's a nuisance, but that's just the way it is.
Once you've got your hands on one all you need to do is use it on your Sinistea and hey presto! You've got yourself a Polteageist! Maybe...
You see, it's possible that a Crack Pot work on won't work on your Sinistea, because there's technically two variants of the Pokémon, not that you'd ever know it.If a Cracked Pot doesn't work, you'll have to check with the vendor every day (again) for the rarer Chipped Pot. This is because your Sinistea is rarer and is the 'authentic' form. Sadly this doesn't make it more powerful, just more irritating to evolve.
But once you've worked it all out, you'll have yourself a Polteageist regardless! And if you had to use the Chipped Pot instead, feel free to flaunt it to your friends because yours is authentic. There's an underlying message here, but we can't quite put our finger on it.