Return To Aether Paradise

Walkthrough: After Gladion having made the connection that The Aether Foundation has been working with Team Skull to retrieve Cosmog, you, him and Hau will all go to Aether Paradise to find and save Lillie and Cosmog. As you search for them, you will be sent on a wild goose chase because of Faba, who is also in on the scheme, thus causing you to have to Battle a multitude of both Aether Employees and Team Skull Members, as well as Faba himself on two different occasions. Luckily Hau and Gladion will assist you with this. (Hau will even give you another Max Revive in the Labs Area.)

You will begin on the Docks of Aether, eventually going to the Secret Labs before having to head up the levels to again find Faba to obtain the key to access the basement. Wicke, the only person not apart of what's being orchestrated, will lend you her knowledge on Cosmog and its ability to summon Ultra Beasts. After finally obtaining the key, you will be able to use the elevator to reach the levels inaccessible at first. After such, you will have to Battle Guzma before finding Lusamine.

After finding Lusamine, she will verbally chastise her children in her current madness for stealing her beautiful Pokemon, for trying to stop her from being with her lovely Ultra Beasts. Ultimately, Lusamine will use Cosmog's power to summon the Ultra Beast from before, even if it means Cosmog might die in the process. After such, as Cosmog has changed to a much more motionless form, you will Battle Lusamine in hopes of stopping her. However, even after defeating her, the Ultra Beast will disappear back through the Wormhole, and Guzma and Lusamine will join it.

All Trainers That Will Be Battled, From Arriving To Lusamine:
Aether Foundation Employee - Level 37 Dugtrio, Level 37 Sliggoo
Aether Foundation Employee - Level 36 Herdier, Level 37 Lumineon
Aether Foundation Employee - Level 37 Shelgon, Level 36 Trumbeak
Aether Foundation Employee - Level 37 Primeape
Aether Foundation Employee - Level 36 Parasect, Level 36 Drifblim, Level 37 Vibrava
Aether Branch Chief Faba - Level 39 Hypno
Aether Foundation Employee - Level 37 Pelipper
Aether Foundation Employee - Level 37 Mudsdale
Aether Foundation Employee - Level 38 Electabuzz
Aether Foundation Employee - Level 38 Magmar
Aether Foundation Employee - Level 38 Ledian
Aether Foundation Employee - Level 37 Muk
Aether Foundation Employee - Level 37 Magneton
Aether Foundation Employee - Level 37 Porygon 2
Aether Foundation Employee - Level 37 Vanillish
Aether Foundation Employee - Level 37 Machoke
Aether Branch Chief Faba - Level 39 Slowbro, Level 39 Bruxish, Level 40 Hypno
Team Skull Grunt - Level 37 Golbat
Team Skull Grunt - Level 37 Raticate
Team Skull Guzma - Level 41 Golisopod, Level 40 Ariados, Level 40 Masquerain, Level 40 Pinsir
Aether Lusamine (All Level 41) - Clefable, Lilligant, Mismagius, Milotic, Bewear

With nothing left to do, the four of you (Lillie, Hau, Gladion and Yourself) will spend the night at Aether and try to regroup in the morning. When you awake, Lillie will have finally worked up enough courage to finally wear some of the clothes she had purchased during her journey, as well as sport a brand New hairstyle (still having Cosmoem Nebby in her new bag).

Gladion will come and give you a Masterball for your help, as well as give Lillie one of two flutes that he thinks will help you summon the Legendary Pokemon that could help you get to Ultra Space to help save their mother. Hau will go his separate way to finish his Island Challenge in order to get stronger, while Gladion takes you and Lillie to Poni Island to get some help from their Kahuna.

Post Storyline
Details: After finishing the main storyline, you can return to Aether Paradise to find the Five Zygarde Cells found throughout the structure, as well as find any items you may have missed last Time around. You will also be able to Battle Faba, and obtain a Gift Pokemon of Type: Null and all of its Exclusive Memory Items from Gladion (Conservation Area). Most, if not all, Aether Employees will apologize for their actions from earlier in the game.

Aether Foundation Faba (All Level 61) - Slowbro, Bruxish, Raichu, Alakazam, Hypno

Items:
X Sp.Atk (Docks) - Southwest Ferry
Rare Candy (Docks) - North of Elevator
TM06 Toxic (Docks) - Far Southwest Docks
Dubious Disk (Entrance) - Defeat Faba (Post Game Battle)
Stick (Entrance) - From Aether Employee
Big Malasada (Lab Area) - From Wicke
DNA Splicers (Lab A) - From Aether Employee
Prison Bottle (Lab A) - From Aether Employee
Soul Dew (Lab A) - From Aether Employee
Full Restore (Lab A) - Southwest Part of Room
Beast Balls x10 (Lab B) - From Wicke (Ultra Beasts Storyline)
Hyper Potion (Outside) - Southern Area
Full Heal (Outside) - Northwest Platform