The Phony Form (if it lacks a mark of authenticity) requires a Cracked Pot to evolve.
The Antique Form (if it has a mark of authenticity) requires a Chipped Pot to evolve.
Sinistea is a small dark purple Pokémon that inhabits a teacup. The teacup itself is mainly sea-foam green with a lacy pattern that resembles a face. It has gold on the base and around the rim, which is chipped in a spot and part of the Pokémon's liquid body is "overflowing". A small arm extends from another broken bit and holds onto where the handle would be, completing the look of the handle. A yellow swirl pattern can be seen from up above.
Antique Form Sinistea has a mark of authenticity on the bottom of the cup. Phony Form Sinistea lacks this mark.
Sinistea are said to be created when a lonely spirit possesses a cold, leftover cup of tea. The teacup that Sinistea lives in is a famous piece of antique with many forgeries in circulation. The swirl pattern on Sinistea serves as a weakness. Should it get stirred, the swirl will disappear, and cause Sinstea to get dizzy. It can absorb the life-force of anyone who drinks it. While it can wait patiently, most of Sinistea's opportunities ends up being short. The reason being that it tastes so bad, Sinistea get spat out immediately.