Diancie is now obtainable in Pokémon Legends: Z-A through a post-game Mystery Gift Side Mission. Here’s the concise path to get it:

• Step 1: Complete the main story to unlock post-game access. This includes finishing the main campaign and seeing the credits.
• Step 2: Claim the Diancite via Mystery Gift. Use the in-game Mystery Gift function to download the item that enables the Diancite Mega Stone.
• Step 3: Start the side mission Shine Bright Like a Gemstone. The mission is triggered from the Looker Bureau after you’ve secured the Diancite.
• Step 4: Complete the Shine Bright Like a Gemstone side mission. You’ll follow Mimi and Emma around Lumiose City and then across the map to unlock Diancie.
• Step 5: Encounter and recruit Diancie. After finishing the side mission steps, Diancie becomes available for capture and addition to your team. If you want Mega Diancie, you’ll also receive the Mega Stone (Diancite) to trigger the Mega Evolution in battle.

Notes and tips:

• You can start the Mystery Gift before finishing the main story, but you won’t be able to obtain Diancie until after you complete the post-game steps. This sequence is commonly confirmed by players and guide outlets.

• Diancie is obtained as a Mythical Pokémon with access to its Mega form via the Diancite stone. The process centers on the Mystery Gift Diancite and the Shine Bright Like a Gemstone side mission.

• If you’re looking for a quick reference, many guides state the same three components: finish campaign, claim Diancite via Mystery Gift, complete the Shine Bright Like a Gemstone side mission to recruit Diancie.

If you’d like, I can tailor this to your exact in-game progress (e.g., which region you’re currently in, what you’ve completed so far) and provide step-by- step prompts with precise NPC locations.