Driftveil City

Defeat the Rotation/Triple Battle against the Motorcyclist with Archen (26), Sigilyph (26), and Tirtouga (26). Note that Embargo prevents held item use for 5 turns.

Hug the market wall to your right to find a hidden entrance. Go down to find TM63 - Embargo.

Head east towards the Driftveil Drawbridge and enter the building at the corner. Talk to the man near the reception desk to receive a Shell Bell.

Head west, past the intersection, and enter the building to the east. On the 1st floor, a girl will reward you with a Heart Scale if you show her a Pokemon with a specific TM move (this can be done once per day for different moves).

Head up to the 23rd floor of the building and talk to the man to receive an Everstone.

Head south to the gate for the Pokemon World Tournament, but you'll be blocked by a guard.

Return to the Pokemon Center, head north through a gap in the fence onto an asphalt road. East leads to the Driftveil City Gym, and west leads to Route 6.

Head up the stairs to the east. You'll encounter an Old Team Plasma Grunt.

Answer 'Yes' to his challenge to battle Trainer Rood.

After defeating Trainer Rood (Herdier 27, Swoobat 27), Hugh will appear. Rood will then offer you Zorua. Accept it if you have space in your party.

The Driftveil City Gym is now open, but it's likely too early to challenge. Head to Route 6.