Eterna Forest has a reputation for being a mysterious place, from the abandoned Old Chateau to the strange Moss Rock. Eterna Forest is a long and complex structure of trees that make it a natural maze, where a lot of people get lost. It's home to many Bug- and Grass-type Pokémon, and is currently the only known place in Sinnoh to find and catch Buneary.
Once inside, a woman named Cheryl will ask you to escort her through the forest, since she is worried about being attacked by Team Galactic. When traveling with her, all wild encounters will become Double Battles! If you want to catch any of the Pokémon found here, it is recommended you do so after you've parted ways; while you're in Cheryl's company you're unable to catch wild Pokémon while there are two enemies on the field, and her Chansey will quickly finish off a single Pokémon. You also have the option of challenging pairs of Trainers as a tag team with Cheryl or by yourself by talking to them to fight individual battles. The greatest benefit of traveling with her is that after every battle, she will fully heal your team! This makes Eterna Forest a great place to train for your upcoming Gym Battle.
The most direct route from the southern exit to the northern exit is as follows: travel northeast from the entrance, through the tall grass, and then directly north, following the path between a Lass and a Bug Catcher. Then, continue east a short way to the trees and then south to a large patch of tall grass. From there, continue east until the eastern wall is reached. A considerable distance north of there is the exit, around a small rock ledge. However, you may (and should) choose to deviate from this route, as there are a lot of items and Trainers to be found in Eterna Forest. Once you reach the northern exit, Cheryl will leave. Upon leaving, Cheryl will give you the Soothe Bell as a gift. Additionally, the Old Chateau (where HM01 (Cut) is necessary to clear the trees blocking it) is near the exit. You'll be coming back here later.
Route 205 (east)
When you come out of Eterna Forest and return to Route 205, all you need to do is walk across the bridge. The Fishermen won't battle you unless you talk to them, so if you don't want the experience just go straight ahead to Eterna City.
Later on, after defeating the Eterna Gym and therefore acquiring the ability to use Cut outside of battle, you can Cut the small trees that separate the northern and southern parts of Route 205 and find more items in this shortcut around Eterna Forest.
Once in Eterna City, you will see that Team Galactic has set up another Headquarters. As usual, you'll be stopping them from their evil work. The building is blocked by small trees, though, so you must first go to the Gym, and battle the Gym Leader, Gardenia. Before that, visit the statue in the northeast corner of the and Barry will join you. As you walk up to the statue, you'll see Cyrus there. Once again, he will make a reference to Sinnoh's legends.
After Barry and Cyrus leave, first look carefully around the back of the statue for a hidden Draco Plate. As you head back the way you came, you'll meet Cynthia who gives you a copy of HM01 (Cut)! Also, talk to the lady in the Pokémon Center for a Pokétch app called Friendship Checker which allows you to tell how much the Pokémon in your Party like you. This is useful when trying to evolve Pokémon that evolve with high Friendship.
Here are some other places you can check out in Eterna City:
There are two potential types of fossils that can be mined in the Underground, but only one type is available per game. It has to do with the last digit of your Trainer ID number (visible on your Trainer Card). If the last digit is odd then you'll be able to obtain the Skull Fossil (Cranidos), but if it is even then you'll get the Armor Fossil (Shieldon). Fossils can be revived into their respective Pokémon in the Oreburgh Mining Museum. The Fossils from Generation I and III can only be obtained after getting the National Pokédex.
The Herb Shop is a shop that sells herbs with effects similar to other medicines. They're relatively inexpensive, but their bitter taste lowers the Happiness of Pokémon they are used on. Revival Herbs are probably the most useful thing that can be bought here.
Obviously, this is a store where Bicycles are sold. However, you can't purchase a bike from the store. The owner is being held captive by Team Galactic!
You can challenge the Eterna City Gym if you want but if you need more time to train or find new Pokémon you can check out the west side of Route 211 and part of the north side of Mt. Coronet by heading through Eterna City's eastern exit. See this section for more details on Route 211 and this section for Mt. Coronet's North Area.
Interior of the Eterna Gym
This Gym specializes in Grass-types. Be aware that the highest-leveled Pokémon you'll encounter here is Level 22 and quite powerful. Gardenia's Grass-types are weak to Fire- and Flying-type moves. Avoid bringing Ground-, Rock-, and Water-types as these types are very weak against her Grass Pokémon. If your starter was Chimchar, this should be an easy fight. If not, the common Pokémon Starly evolves into Staravia at Level 14, and is a good Flying-type that should be considered having on any team, regardless of your starter.
Once inside the Gym, you must find and battle three Trainers before Gardenia will face you. The hands of the giant clock in the middle move every time you defeat a Trainer. When they are pointing north, south, east, or west, you can walk across them to reach other areas of the Gym.
First she will send out her Turtwig, which will use moves like Sunny Day and Reflect to help set up the rest of her Pokémon. However, Sunny Day also powers up Fire-type moves, which you can use to your advantage. Gardenia's entire team knows Grass Knot, an attack that does more damage to heavier Pokémon. If Turtwig had a chance to set up Sunny Day, Gardenia's Cherrim will have its Attack and Special Defense increase by 50% due to its's Ability. Try to knock it out quickly so you don't get hit by Leech Seed, which will drain your HP to heal the opponent. Gardenia's highest-leveled Pokémon, Roserade, has an additional Poison type that makes it weak to Psychic-type attacks. Along with the strong attack Magical Leaf, Roserade also has Stun Spore, a move that can paralyze your team, and Poison Sting, a weak attack that can poison your Pokémon.
Once you beat her you will receive the Forest Badge, TM86 (Grass Knot), and the ability to use Cut outside of battle. Now, you may enter the Galactic Eterna Building.
Once inside the Team Galactic Eterna Building, Looker will approach you again. Several staircases in this building lead to dead ends, where Galactic Grunts and items are located.The correct staircases have a poster near them. Defeat the two grunts guarding the left-hand staircase, then head upstairs. Fight the lone Grunt, collect the X Speed behind him, then head back downstairs and up the right-hand staircase.
Galactic Eterna Building 2F
Back on the second floor, make a quick detour up the right-hand staircase, defeat the Grunt and grab the X Special that's partially hidden by the appliance behind him, then head back downstairs. Before heading up the middle-right staircase, talk to the Grunt working at a computer for another battle.
Galactic Eterna Building 3F
Battle the Scientist, then head up the right-hand staircase that leads up to 4F; you'll find another dead end but you can also collect a Blue Shard and a Revive. Head back downstairs and up the middle-right staircase to the top floor.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Before heading to the shop, visit the Pokémon Center and make sure that your party has a free space - just before arriving at the Bike Shop you'll encounter Cynthia again, who will give you a Pokémon Egg that hatches into a Togepi. When you meet Rad Rickshaw at his shop, he will give you a bicycle. You can ask him to read you the manual for the bike here once you receive one. A unique feature of this bicycle is that the player can press the B button to switch between the third and fourth gears. Fourth gear is faster than third gear and allows the player to ride up muddy slopes and jump further from bike ramps. Third gear can jump bike ramps, but does not jump as far. Head to Eterna City's south entrance - now that you have a bicycle, you're allowed onto Route 206, the Cycling Road.
Finally, before you leave down, make sure you bring Pokémon which know Cut, and TM70 (Flash) (which is found in the basement of the Oreburgh Gate) and Rock Smash, as you'll need them to enter and navigate a few of the upcoming areas.
Now that you have Cut, you can visit the Old Chateau, which is located in the northern part of Eterna Forest, by the exit. Buy some Nest Balls from the Pokémart in Eterna City then backtrack to Eterna Forest, because the Old Chateau is home to Rotom. Since there are no in-game Trainers who use Rotom, you will need to face it here at some point for it to be counted as seen in your Pokédex. You'll find Gardenia near the entrance who, when you talk to her, appears too scared to go in due to the ghosts. Rotom aside, the Old Chateau is worth exploring to collect the items inside it. Go up the stairs to find the hallway with multiple small rooms. The only wild Pokémon you'll encounter here are Gastly, so you might want to bring some Repels to save yourself some annoyance.
From here, use the first door on your left to reach a room with a TV. During the night (8:00 PM to 3:59 AM), Rotom can be found here, hiding in the TV. Unlike in Diamond and Pearl, where you could only find it once the National Dex has been obtained, it can be found and captured right now.
Head upstairs and into the main hallway, and save the game before interacting with the TV which Rotom has possessed. Rotom is not very tough, so take care to whittle its health down with flying-, bug-, or poison-type moves. As Rotom is only level 20, Nest Balls will have the same catch rate as Ultra Balls in this encounter, so send it to Sleep or Paralyze it, then use your Nest Balls to catch it. If you got the Dusk Ball from Oreburgh City, then that will work even better than Nest Balls.