Note: This portion of the Relic Castle is only accessible after you have met N in the Dragonspiral Tower.
As you try to walk down the stairs into the Relic Castle, Cheren will stop you and say Alder is already inside. Enter the Relic Castle.
Things to know if you haven't already been here before:
-You will fall through the quicksand to the floor below if you run over the quicksand or walk into the center. To avoid falling through, simply walk over the quicksand and avoid the center.
-You can get a fossil from the female Backpacker on the first floor: Cover Fossil (Tirtouga) or Plume Fossil (Archen).
Walk to the west end of the room, past the Worker that previously blocked you, and go down the stairs.
Ryoku, a Team Plasma Sage, is at the bottom of the stairs. He says Ghetsis wants to test your strength again. Walk past him and battle with the Plasma Grunt.
Fall through the quicksand west of the female Plasma Grunt.
Cheren will land after you. Alder will call you over and Ghetsis will essentially restate Team Plasma's goals, as if you hadn't heard them enough already. Ghetsis leaves and the three of you go outside. The Xtranceiver will eventually ring. It's Professor Juniper and she tells you to go to the Nacrene City Museum immediately.
Alder, Cedric, Professor Juniper, and Bianca are all standing in front of the museum. Lenora comes out of the musuem and reveals why Team Plasma stole the Dragon Skull earlier. They were actually looking for the Light Stone in order to resurrect the legendary Dragon-type Pokemon. She gives you the Light Stone and Alder tells you to go to Opelucid City because Drayden/Iris might have more information. Fly to Icirrus City and go to Route 8 at the southwestern part of town.