Trainers want to know if they can Mega Evolve their Pokemon in Pokemon Brilliant Diamond and Shining Pearl.
Nintendo announced its Sinnoh remakes would be entirely faithful during their initial debut. This, for better or worse, turned out to be wholly true, with several features being left out in order to maintain that standard. Fans reaction to this news was mixed with some missing the combat-specific or quality of life features they'd previously enjoyed in recent titles. One of the features in question was Mega Evolution.
Mega Evolution in Pokemon Brilliant Diamond and Shining Pearl
No, trainers do not have access to Mega Evolutions in Pokemon Brilliant Diamond and Shining Pearl. Since the two are "faithful remakes" of their original titles, and Mega Evolution was not available at the time of their initial releases, Mega Evolution was not implemented into the new games.
Mega Evolution first appeared in the Kalos region with Pokemon X and Pokemon Y. When both the Pokemon is holding its type-appropriate Mega Stone and the trainer is wearing their Mega Ring, trainers can prompt Mega Evolution with their Pokemon in battles.
Like Dynamaxing from Pokemon Sword and Shield, Mega Evolution is a temporary state that changes a small selection of attributes about the Pokemon. Dynamaxing changes the size and available moves while Mega Evolution changes their appearance, type, and potentially their abilities. Both increase the chosen Pokemon's strength.
Dynamaxing is also not available in the Sinnoh region, nor does it appear in any locations outside Galar.
Trainers can pick up Brilliant Diamond and Shining Pearl for the Nintendo Switch.