First, there's some more hidden stuff to find in the previous areas by using Surf and Strength. There's some nameless old guy looking for a legendary pokemon in a cave near Mistralton City. When you find him you get a bunch of unexpected backstory involving the origin of pokemon.
I see you're quite skillful for your age... Perhaps you could... hrm... It's a possibility, I suppose... Well? Will you lend your ears to this old man's ancient story?
It was long ago, when people and Pokémon lived in their own separate worlds... Through a war humans started, a forest in which Pokémon lived was engulfed in flames.
The Pokémon could not escape through the fire and smoke... There then appeared the legendary Cobalion, Terrakion, and Virizion! Terrakion used its characteristic strength to destroy the boulders blocking escape. Virizion made use of its swiftness to protect Pokémon from the sparks. And Cobalion guided the frightened Pokémon out of the forest.
Cobalion, Terrakion, and Virizion fought away the warring parties with overwhelming strength. The parties, frightful of the power the three showed, at last put an end to their war...
If people do not cease fighting, Pokémon can have no peace. The trio knew that people's deeds could cause the loss of many lives... To avoid contact with people, they went away to places unknown... Since then, the three have almost never been seen, and they thus turned to legend...
I have been searching for them for countless years on end... And at last, I have ascertained there to be one deep in this cave. But the legendary trio has no trust in humans. If a person draws near, they will bare their fangs and attack. They only know the world as it was so long ago, so it cannot be helped... They remain ignorant of the fact that our world has become one of people and Pokémon understanding and helping each other.
Perhaps if you could make them realize that people and Pokémon can live together in trust... Even they could choose to live with people if given a reason to reconsider them...
Who knew pokemon could be this deep? I feel like I'm reading Princess Mononoke... stuff about humans and intelligent forest creatures at war with each other...
The end of this cave has Cobalion.
Through you, Cobalion seems to have realized that the world has changed into one where people and Pokémon live helping each other... Cobalion seems to desire to live together with you.
If Cobalion's heart was softened through fighting you, Terrakion and Virizion, whom he shares a strong bond with, should also know soon. I think they, too, will want to evaluate your heart.
The obstacles keeping people away from Terrakion and Virizion should be gone now. Finding them, however, is up to you... But they are undoubtedly awaiting you...
After going to route 11 right of Opelucid City, you get to Village Bridge. A very strange construction where the bridge goes up above the ground, and rows of houses are built ontop of it. Underneath the bridge, the river is mostly gone up and there's tall grass and trainers everywhere, and there's this dude under the bridge that starts singing some folk song when you talk to him.
Koontz: Singing gives life to my spirit. Will you listen to the voice of my spirit?
Sounds like something out of Animal Crossing.
This bridge leads to Lacunosa Town, a place where everyone's obsessed with following scrict rules and schedules. This snoozing old lady rambles on about something...
W-what? What do you want, now? Did you pick now to listen to my stories? Mmm... OK, I'll tell you...
Behind our Lacunosa Town, there's a big hole in the ground. That hole, way in the past... Aaaahhh, I'm so tired... That hole, there was a... a big... crashed down... inside was... big, scary... really scary...
Zzz...
Uh... all I got was something big and scary, in a hole, way in the past... reminds me of Earthbound... yet I can't find anything like that anywhere around this town... could be just an NPC saying random stuff for no reason?
Past Lacunosa is route 13, which just has more grass, more trainers, and a ton of items to find. Almost all the pokemon around this side of Unova are from previous generations. The whole game up until the elite four has been nothing but gen 5 pokemon.
Ahh, the classic "6 magikarps" fisherman trainer is on this route. How fitting.
There's this one trainer with a Wingull who says the Wingull is looking for 3 grams... what could these "grams" be referring to, hmm? In any case the grams are just random pickups you find along the route.
Okay I gave the wingull its 3 grams in exchange for TM89 U-turn.
Got the splash plate and draco plate from a guy in a house very far off the main path... these plates were treasures resting in a sunken temple near Undella Town... which is the next area after route 13.
Undella Town - A Town of Rippling Waves
This girl sings to me about the sea, the source of life... and gives me HM06 Dive. I can see where this is going... but wait...
Oh wow the first house I go into has Cynthia, the champion from Diamond/Pearl in it.
She's a far more likable character than Alder (oh boohoo you lost your favorite pokemon and can't fight to your full potential anymore, this guy's supposed to be a champion?). She introduces herself and explains what she's doing in Unova.
What's this? What an uncanny resemblance... It's surprising to meet another trainer who has eyes like that pokemon trainer. What's your name?
...
OK. I'll remember that! Warpio, nice to meet you. I'm Cynthia. I'm a pokemon trainer, too, like you.
I have an insatiable curiosity for researching pokemon myths. I'm sure you know about Undella Town's Abyssal Ruins, right?
I'm staying here at my friends villa so I can investigate them. In order to get to know each other better as pokemon trainers, I would like our pokemon to have a match.
Before I send out my pokemon, my heart always begins to race... Interesting... my pokemon in their poke balls are radiating a happy feeling. Are you the reason? What are you?
Holllllly shit her pokemon are level 75 and up, and Spiritomb double-team'd on me, I couldn't even get a single hit in.
... welp, Cynthia crushed me. I'll just be ignoring that for now. So it looks like the ruins under the sea are my next destination...