Pokémon Adventures: Diamond and Pearl/Platinum Vol 8

From the VIZ Media edition

To prepare for their big battle, our trio of friends each need to reach out for help - from Gym Leaders to new Pokemon. Then Lady, Pearl, and Diamond are finally reunited in the very heart of Team Galactic's headquarters. Can they release the three Legendary Pokemon before it's too late? And who will take on Team Galactic's powerful leader, Cyrus?! Plus, meet Electivire, Honchkrow, Chingling, Spiritomb, Dialga and Palkia!

Source: Anime News Network Encyclopedia