Pokemon Sword and Shield's Galar region is inspired by the United Kingdom, and in that spirit, the games introduce a ghost-type Pokemon that's themed after tea. Sinistea, a teacup, evolves into a whole pot of tea called Polteageist. As far as Pokemon names go, they're some of the best. But they're also cool Pokemon, and it's not too hard to get them if you know where to look and what to do.
Sinistea can be found in Glimwood Tangle, the cool mushroom forest between Stow-on-Side and Ballonlea. You'll have to rustle through the tall grass to trigger an exclamation point to pop up, and Sinistea is among the Pokemon that can show up when the battle starts.
Once you catch a Sinistea, head back to Stow-on-Side and talk to the right-hand man running the marketplace stand. He sells a different "bargain" item every day, most of which are evolutionary items for various Pokemon. The item you're looking for is the Cracked Pot--if he's not selling it today, ask him for his next bargain to see if it'll come tomorrow. One you get it, all you have to do is use it on Sinistea the way you would use an evolutionary stone to get Polteageist.
Polteageist's signature move is Teatime, which forces all Pokemon on the field to eat their berries at once. This can be useful if you think your opponent's Pokemon are holding berries that reduce the damage they'd take from a super-effective move, for example--that way, you can hit the Pokemon head-on for full damage right off the bat.