Shiranui got more information about the Diamond Domain from Xeimei including the fact it was ruled by a Pokémon known as Diancie.  "[i]Diancie, the Jewel Pokémon.  It can instantly create many diamonds by compressing the carbon in the air between its hands.[/i]"  He let Majin back out and got the Hoopa to create a portal there, which Shiranui and company jumped through.

Shiranui saw many Carbink and he scanned them.  "[i]Carbink, the Jewel Pokémon.  Born from the temperatures and pressures deep underground, it fires beams from the stone in its head.[/i]"  One of the Carbink went over to Shiranui and introduced itself as Dace, translated by Majin.
"Dace is saying the Diamond Domain is in deep trouble from the Heart Diamond losing its power," Majin translated, "He wants to present you to Diancie."
"Alright then, Dace, lead the way," replied Shiranui.

Shiranui was led to the Heart Diamond where Diancie was desperately trying to create a new one.  Diancie's servants—named Bort, Allotrope and Merrick—were shocked to see a human in their midst.  "[i]I bring to you the legendary child of the moon,[/i]" stated Dace.
"[i]Him?[/i]" asked Bort.
"[i]How could a human possibly help our leader out?[/i]" Allotrope added.
"[i]Because he has the very being we're looking for,[/i]" answered Dace.  Majin was translating the whole conversation for Shiranui.  "[i]Moon child, we need Xerneas' help,[/i]" Dace stated, "[i]The Tree of Life is the only one that can help our ruler make a new Heart Diamond.[/i]"
"I'll be glad to help," Shiranui replied with a smile as he let Xeimei out.

Xeimei's Fairy Aura ability gave Diancie a boost in power.  Diancie used the boost to create a diamond that was spherical in shape.  "[i]This isn't the Heart Diamond at all![/i]" Diancie complained.  Shiranui recognised Diancie's creation as a mega stone.
"Then perhaps another boost will help," he replied, activating his Mega Ring.  Diancie was wrapped in an orb of energy and soon a Mega Diancie burst forth in all its radiance.  "You can do it!" Shiranui encouraged.  Diancie used the power it gained from Xeimei's ability and its own Mega Evolution to create a diamond that was as big as the previous one.
"[i]Our Domain is saved![/i]" cheered Dace before glowing and transforming into a second Diancie—a phenomenon similar to evolution, but not recorded as such.
"[i]Moon child, thanks to you, I've created another Heart Diamond,[/i]" said a very grateful Diancie, "[i]I want to thank you by coming with you on your journey.[/i]"
"I would be honoured to have you on my team," replied Shiranui, presenting a Dusk Ball.
"[i]Dace, since you've now transformed to a Diancie like me, I'm abdicating my rule,[/i]" Diancie decreed, "[i]As of now, you're the new ruler of the Diamond Domain![/i]"  Bort, Merrick and Allotrope were happy for their old friend as Shiranui tapped Diancie with the Dusk Ball.  The ball immediately snapped shut.
"[i]Moon child, you will always be welcome here,[/i]" stated Dace, "[i]Good luck on your journey.[/i]"
"And good luck on your reign, Dace," replied Shiranui as Majin conjured a portal to the Pokémon League.  Both Shiranui and Majin stepped through the portal and Shiranui recalled Majin before looking around for Liam.