Question

How to Transform Carbink into Diancie? (In Pokemon Ultra-Moon 2021)?

Comments

That Dex entry is referring to Darwinian Evolution, 1 species slowly becoming another over long periods of time, rather than Pokemon Evolution.

Answer 1

You can't. Carbink doesn't evolve into Diancie, Diancie can't de-evolve into Carbink, and nothing in between can happen either. The Pokedex says that Carbink suddenly transforms into Diancie sometimes, but there's no way to achieve this legitimately in any game to date.

Hope I helped!