After dealing with Team Galactic for a long time, it's time to face the head of the organization. Of course, this won't be your last meeting with the group, but it's a good step. Head to the Team Galassia building in Pietravela and make your way through the puzzle to reach the boss. Here's how to find Cyrus in Pokémon Brilliant Diamond and Brilliant Pearl.
Before you can go through the maze that is Veilstone's Galactic Building, you will first need to obtain the Galactic Key from the Team Galactic Warehouse on the other side of town. After obtaining the key, return to the Galactic Building in the northeastern part of the city. When you arrive, take the left or middle door.
Inside the reception area, use the Galaxy Key to access the second floor stairs. Once you are on the second floor, follow the corridor and take down the Team Galaxy recruits who are waiting for you. Afterwards, continue down the corridor until you reach an opening. Inside the room there is a warp. Use it to progress.
The warp pad will drop you off near some scales. Go up the stairs and follow the corridor. This will take you to a set of three warp pads. Take the warp pad on the left. This will leave you near another flight of stairs. Grab those and you'll be in front of the boss door. Use the galactic key on the door to open and enter to face Cyrus.