Evolve Diancie into Mega Diancie

Requirements

You need three things: a Diancie, a high Friendship level with that Diancie, and a Nether Star. Raise your Friendship by battling alongside Diancie, healing it when it faints, and keeping it in your active team. The higher the Friendship, the better.

The Mega Evolution Process

Once Friendship is sufficiently high, give the Nether Star to Diancie by placing it in its held item slot. Diancie will transform into Mega Diancie. The transformation is permanent until you remove the Nether Star.

Reversing the Mega Form

Unlike traditional Pokémon games, this Mega Evolution is fully reversible. Simply take the Nether Star away from Mega Diancie, and it will revert to its standard form. You can repeat this process as many times as you want—the Nether Star is never consumed, so you won't lose it during the transformation.

This makes experimenting with Mega Diancie risk-free. Switch between forms whenever you need to adapt your team for different battles or situations.