Team Plasma's Gone Too Far

Slide across the ice north, then north again to a small patch of non-icy ground.

Move to the north tip of this patch and slide west to land on non-icy ground with a female Team Plasma Grunt. Talk to her to start a battle. She uses a Level 44 Watchog (Normal) and a Level 44 Muk (Poison).

Slide west until you are in front of the Gym. Move through the gap between the ice crystal and the building on the east side of the path leading south, then slide south.

You will slide past the last Grunt. Move to the west side of the non-icy patch you are on and slide north onto the non-icy patch with the final Grunt. Talk to her to start a battle. She uses a Level 44 Seviper (Poison) and a Level 44 Weezing (Poison).

After defeating the first two Grunts, slide west until you are in front of the Gym. Move through the gap between the ice crystal and the building on the east side of the path leading south, then slide south. You will slide past the last Grunt. Move onto the west side of the non-icy patch you’re on at the moment and slide north onto the non-icy patch with the final Grunt on it. Talk to her to start a battle. She uses a Level 44 Golbat (Flying/Poison) and a Level 44 Garbodor (Poison).

Once all three Grunts are defeated, head north back to the Opelucid City Gym. It is recommended to heal your Pokemon at a Pokemon Center first.

Talk to Zinzolin, who is standing in front of the Gym, to begin a battle. He uses a Level 46 Cryogonal, a Level 46 Cryogonal, and a Level 48 Weavile (Dark/Ice).

After defeating Zinzolin, Drayden will appear and speak with you. He will enter the Gym and then reappear, giving you a key item.

A member of the Shadow Triad will appear, followed by two more. They will disappear with the key item.

Follow Drayden, who will stop partway back to the Pokemon Center, confused.

Continue back to the Pokemon Center, heal your Pokemon, then head east towards the gate back to Route 11.

Just before the gate, a member of the Shadow Triad is waiting. Move up to them and begin the battle. They use a Level 46 Pawniard (Dark/Steel), a Level 46 Absol (Dark), and a Level 48 Pawniard (Dark/Steel).

Drayden will catch up after the battle, but the Shadow Triad member will teleport away.

You will receive a call from Hugh and Cheren. Cheren will then appear in person and talk to you before running off again.

After talking with Drayden, fly back to Undella Town.

Heal your Pokemon in Undella Town, then head west towards the Marine Tube entrance.

Just before reaching the Marine Tube, you will receive a call from Hugh. Speak with him, then enter the Marine Tube.