Sorry, in lieu of an official name for the 7th (and final) island trial, we just couldn't resist. Past the captain's gates, you'll immediately be alerted to an ancient stone monument. If you wish to continue, you need to accept the island trial.
Here goes nothing... Place a Pokemon that's good against Dragon-types at the front of your party. Next, proceed forward past the stone monument. Halfway between the monument and the alcove ahead on the east side of the canyon wall, you'll be alerted to a piercing gaze.
Continue forward and you'll be ambushed by a wild Jangmo-o. Although there's no trial captain to watch you, you can't catch any Pokemon here. Hit it with super-effective Ice, Fairy or Dragon-type moves to quickly send it scurrying.
After that surprising encounter, go inside the alcove to the north-east. Inside, you'll find TM02 Dragon Claw . Return to the main path and continue following it north. As you approach the small rock by the west side of the canyon wall, you'll be jumped by a Hakamo-o.
The middle evolution of Jangmo-o has gained the Fighting-type, so it takes a whopping 4x damage from Fairy-types and the usual 2x damage from Flying, Psychic, Ice and Dragon-types. We wouldn't bet against the fairies. Hmm, it's pretty obvious what's going to happen next.
Carry on ahead towards the pedestal with the Z-Crystal. Try to remove the Z-Crystal and you'll be met by the leader of the pack. Nope, this is nothing like a scene from a Monster Hunter game.
It knows Clanging Scales, a new Dragon-type special move that deals a lot of damage and lowers Defence with next to no drawbacks. In addition, it has Sky Uppercut for Fighting-type STAB and Flash Cannon to punish Fairy-types. It can also stall with Protect.
Whenever it's alone, it may call ally Hakamo-o. These dragons aren't immediately threatening, but can be a nuisance with Sky Uppercut. Otherwise, it can be aided by the Bug/Steel Scizor as well. These ones aren't too scary either, but have Metal Claw to mess up Fairy-types.
The easiest way to approach this battle is with a high level and/or speedy Fairy-type to deal 4x damage to Kommo-o. However, with its aura boost and good defences, you may not score a OHKO, which leaves your Fairy vulnerable to Flash Cannon.
Otherwise, Psychic or Flying-types can do quite well, but they'll need to be durable to eat a Clanging Scales. Ice-types suffer the same issue as Fairy-types, but only hit for 2x damage, while Dragon-types will be severely punished by the powerful Clanging Scales.
Triumph against the scaly beast and nothing will stop you retrieving that Dragonium Z , which proves you've completed Poni Island's island trial. Now that you've completed all seven of Alola's island trials and beaten all four island kahunas, you're technically the Island Challenge Champion.
However, it's not quite that simple. In any case, you can worry about the formalities later. For now, proceed past the pedestal and exit to the far north.