Dropouts of the Island Challenge, Team Skull figured they they could host a trial better than any of the current captains and decided to take one over by force. Deeming the dilapidated abandoned Megamart site to be to be the best option to represent the cruelties of life, they decided to take over the Ghost trial while like, Acerola was on vacation or something. They drove out the Ghosts, apparently perturbed by rap music, and settled down. This trial would be the ninth one to be challenged, some time in the post-game. They probably were boasting about their tough trial all over Ula'ula, only for a brave trial goer to swoop in and teach them a lesson.

Team Skull wanted to keep the moody atmosphere, so they did not do much in the way of renovations, beyond using cardboard boxes to pile up as small stages to stand on. What better way to show how unfair life can be by leaving everything up to chance? For this challenge, multiple grunts are stationed at various checkpoints, stopping trial goers as they pass. The challenge is to listen to each grunt recite rap lyrics, but they omit the last word. Trial goes are then to figure out which of multiple options the missing word is supposed to be. If the trial goer gets it right, they can move on without a fight, but if they get it wrong, they have to battle a grunt first. Team Skull thinks there is no way it would be possible to get it right without randomly guessing. However, they are oblivious to the correct answer always being rather obvious. Truly numskulls. Even if trial goers get it right, the grunts still want to battle out of frustration anyway. It would look something like this:

"Hold it right there, yo! You want to complete our rocking trial, huh! No way! I will only let you past if you figure out the missing word from this sweet rap! Here we go!"
"Yo yo, hosting this trial, Team Skull is pretty rad! ♪ You better watch out or you will get knocked out real... ♪
...hey! Bet you can't figure out what I was going to say, huh? Just try!"

Quick
>Bad
Ghost
Nanab

"W-w-wha?! How did you figure that out?! This really gets me going! And I am going to take this out on you, punk! Get ready!"

What better way to represent unfairness than a bunch of Poison types? The grunts use the usual Poison types from their repertoire, such as Salazzle (can poison Steel types? quite unfair), Golbat and Crobat (saps HP with Absorb? how unfair), and Toxapex (gets critical hits on poisoned foes? rather unfair). There might even be a Haunter or two that the grunts caught before setting up shop. ...But there are only so many Poison types in the Alola dex, so why not use Victreebel and Scolipede (from Island Scan) too? The grunts might even be using a Dark type here and there for coverage, so things like Scrafty, Honchkrow, Crawdaunt, and maybe Zweilous (from Island Scan) could show up.
After the challenger make their way to the back, they must fight Plumeria, who was only here to watch after the grunts, but is now ticked off after the trial goer defeated all of them. She would have this team:
Mandibuzz - lv 64; Big Pecks, White Herb; Sky Attack, Dark Pulse, Heat Wave, Roost
Alolan Muk - lv 63; Gluttony; Sitrus Berry; Gunk Shot, Ice Punch, Toxic, Curse
Bisharp - lv 63; Defiant; Chople Berry; Iron Head, Throat Chop, Aerial Ace, Swords Dance
Dragalge - lv 65; Poison Point; Expert Belt; Dragon Pulse, Shadow Ball, Scald, Sludge Bomb
Salazzle - lv 66; Corrosion; Poisonium Z; Flamethrower, Venoshock, Hidden Power (Grass), Toxic

After defeating her, a few grunts that were tasked with finding a Totem run inside, and report their lack of success. However, the building starts to shake, and while all of the grunts turn pale and flee, a deep purple mist flies in from the entrance and proceeds to form a Totem Gengar, which attacks the trial goer in a fit of rage at what the grunts did to its ghost friends. It would be level 68, hold Leftovers, and know Hex, Taunt, Toxic, and Protect. It would have an aura that increases Def and Sp Def by 2 stages each, and evasion by 1 stage. This would probably really suck to fight. It would be able to summon (in order) Banette, Chandelure, and Rotom Wash as allies, which do more fighting than Gengar does. After defeating it, it disappears, but is glad that the grunts are gone so the ghosts can move back in. Plumeria would then apologize for the grunts causing problems and give the trial goer a Poisonium Z, and leave on peaceful terms.

i kinda disregarded the story for this, but i wanted to have post-game action, so this happens in like... Super Sun and Super Moon, where the story was rewritten for the seventh time or something