Undella Town is an elegant place to beat the heat, and a handful of villas line the beach. During the summer months, the town is a hot vacation spot. The large villa on the west side of town is home to a wealthy family of Trainers who are always looking for a tough opponent. When you are done in town, wade out into the water to reach Undella Bay.
On the plateau to the west of the Pokémon Center is a girl, staring out to sea. Speak to her to obtain the final HM of the game, HM06 (Dive). There aren't many places where the move can be used outside of battle, but it will be important to have when hunting for treasure in the underwater ruins.
Meet Cynthia, Champion of Sinnoh
In the first villa to the west of the Poké Center is where you meet Cynthia, Champion of the Sinnoh region. When you first meet her, she will battle you regardless of the season. After that, she will only battle you during the spring. Her Pokémon are very high-leveled, so train against her team while you can. Her Pokémon will aim for your weaknesses, so be sure to do the same to them. Just remember that her Spiritomb and Eelektross have no weaknesses. Normal-, Fighting-, and Psychic-type attacks have no effect on Spiritomb, and Ground-type attacks don't work on Eelektross.
The large villa in the west belongs to The Riches, a wealthy family. Talk to the youngest member, Draco, outside and he will challenge you to a battle. If you stop by on another day and talk to him inside the villa, the number of family members you can battle will increase every time, until you can fight all six. After defeating Draco the first time, you will fight each of them with no break in-between. When you finally defeat Miles, the head of the family, the challenge is over. The Riches give out a lot of prize money anyway, but if one of your Pokémon has the Amulet Coin, you'll end up with a ton of cash!
Inside the large villa's west room is the billionaire father of the family. Rather than battle you, this collector offers to buy your rare items, like the flutes you get from the treasure hunter on Route 13. Talk to him when you get these items.
Visitors flock to Undella Town to swim in the beautiful Undella Bay. There isn't much of interest on the surface, but if you explore beneath the waves, you'll find the mysterious Abyssal Ruins spread out over the seafloor. Dive down into one of the four spots of darker water and enter the submerged ruins.
The Abyssal Ruins are believed to be the remains of an ancient temple that sank beneath the waves. No one knows who built it or why, but an explanation may lie with the odd symbols etched into the walls. When you dive down to the ruins, hold down the Control Pad as you descend, or the currents will take you back to the surface. To be fully prepared, you need a team with Dive, Surf, Strength, and Flash.
Each of the four dive points connect to the same large room on the first floor. All items can be reached from any entrance. Upon entering the rubble, the sound of a switch can be heard. You will only be able to explore about 500 steps before the currents rush you back to the surface. To reach the second floor, you must reach the center of the room in 190 steps or less. Inspect the purple pillar there and it slides out of the way. Behind it is a spot where you can float up to the second floor.
The second floor is a lot smaller than the first. Swim to the south and take the second right. Work your way up to the northeast corner, and swim along the north wall. Head south then west to reach a purple pillar; use Flash directly beside it and it will move out of the way. Go either north or south to reach the third floor.
There's almost nothing here, except for a few more purple pillars and a single item in the middle of the room — the Relic Crown. This is the last of the ruins' treasures, so just wander around a bit until the currents wash you to the surface. All totaled, the relics housed here can be sold to the collector in the large villa for a combined 1,706,000!
Route 14 is a wooded multi-level area with a majestic river flowing west to east. The spray from the river turns into mist, so the route is almost always covered with fog. The Abundant Shrine lies in the trees to the west, and you'll need access to Surf and Waterfall to navigate the entire route.
Leave the sands of Undella Town behind and head south across the first bridge. Climb the waterfall near the second bridge to find an Ultra Ball at the top. Go south across the water and through the tall grass to find a Reaper Cloth.
Climb the waterfall west of here to find Giallo, the last of the Seven Sages. He speaks to you briefly before handing over TM08 (Bulk Up). Looker arrives and arrests him, and as he leaves he mentions that someone has sighted a person matching N's description on a dragon Pokémon in a faraway land, and leaves to investigate.
Go west past Hiker Jebediah and ride down the waterfall. Climb out on the west bank to reach the Abundant Shrine.
The Abundant Shrine is comprised of the remains of an old village. People no longer live here, but the land still shows the remnants from that time. A small shrine stands at the top of the hill, and is dedicated to a Pokémon that used to protect this place.
Go west from the entrance to find TM35 (Flamethrower) on the far side of the pond. Near the second field north of the entrance is a Hyper Potion, and all the way to the west is a Razor Fang. East of Youngster Lester is a Rare Candy. North of here is the shrine.
If you have caught TornadusB or ThundurusW, you will find a group of kids arguing about Landorus. According to legend, Landorus punished Tornadus and Thundurus for damaging property across the region with their strong storms. If you bring the two troublemakers to the shrine, Landorus will appear. Start off with Water attacks to lower its health quickly, then wear it down with Fighting-, Poison-, or Bug-type attacks. Keep tossing Ultra Balls until you catch it.