World of Final Fantasy/Chapter 5: The Champion Who Saved Grymoire

Cornelia
A cutscene will play as soon as you return, during which Princess Sarah decides that a mutual aid treaty between the neighboring anti-federation kingdoms should be put in place, and the Warrior of Light mentions a resistance group called the League of S, and notes that a few members can be found in Saronia. He offers to go meet with them, but Sarah vetoes it, saying the path is full of dangerous Mirages. Naturally, Reynn and Lann offer to go instead, because "we've got tons of Mirages to collect". Selfless as always, Lann. Sarah hands you a letter with the Cornelian Seal, denoting your status as emissary for the kingdom, and sees you off. Your next stop is Saronia, which requires another train ride. Luckily, there's no cutscene this time. Before you go there however, you should return to Nine Wood Hills and visit the North Promenade for several surprises...

The Girl's Tearoom
After a short cutscene, you will be whisked away to a mysterious room, where you will meet the Girl Who Forgot Her Name. She will tell you she had a dream about this, and then explain that you can jump into the shoes of others to help fight powerful Mirages here. For once, even Reynn isn't aware of what's going on. Either way, she asks you if you are ready.

In this area you can spend your Arma Gems to buy Champion Medals, allowing you to summon the various Champions you've met, as well as to unlock ../Intervention Quests/, where you will secretly come to the aid of various characters you've met to help them fight a powerful battle.

For now, you only have access to one Champion Medal, the Warrior of Light, but your Gems are much better spent on the quests. Not counting DLC, you have access to only one quest, "The Girl Who Forgot Her Name".

Head back to Plaza 99 and enter the North Promenade to be whisked away to another location.

The Coliseum
During this short cutscene you will meet the Tonberry, who runs the ../Coliseum/. In this area you can fight against powerful Mirages sometimes not found anywhere else. You will not gain any XP for winning these fights, and you will start with full HP and AP, but your Champion Gauge will start at 0. Whatever you had before the fight will be returned afterwards. Not including DLC, for now the only fight available to you is the "FF Mascots" fight, available in a level 11 and a level 31 form. More fights will unlock as you progress through the game.

Pyreglow Forest
This is a very straightforward dungeon, but has many short forks branching off from the main path. Keep in mind that you will need a Mirage with the "Sizzle" ability to collect all the treasures, and currently your only options for that are the Black Nakk and the Cockatrice. At the end of the dungeon you'll have a Save Crystal, so it's worth it to spend some time leveling up around here. The coming boss is weak to Ice and Water, in that order, so it wouldn't be a bad idea to train up a Deathskull for its "Blizzard" and a Dualizard for its "Water". When you are ready, head towards Forest Lake.

The Secret Area can be reached by following the trail of light on Branch 4, and will take you to an encounter with a Magic Jar. Currently, you probably can't imprism it, due to it needing an Elixir. You can still beat it to gain the Memento, and then return later to imprism it once you have one. Don't worry, the field encounters in Secret Areas respawn every time the dungeon is re-entered.

Forest Lake
This area is very small, just two screens. Make any final preparations you need at the Gate, and go past it to be treated to a cutscene, where you meet Yuna, who promptly accuses you of being Heralds sent by the Federation, summons a Valefor, and then launches you into a boss fight.

This fight is more of a nuisance than anything. Yuna likes to cast Cure, Focus, Shell, and Protect while they are stacked, which you likely don't have any way of dispelling currently. Yuna is no huge threat by herself, so your best bet is to try and keep them unstacked and then hammer the Valefor with Water and Ice attacks.

If you are playing the Maxima edition, or have the DLC Mirages unlocked, you have a wide variety to choose from for this. Sky Dragon, Kaguya Flan, Red Bonnetberry, 2P Serafie, Quacho, Quachacho, Miney, and Mo can all exploit these weaknesses. If not, you're likely only going to be able to exploit it with Deathskull, Dualizard, and potentially Bablizz if you picked it up.

After you win the fight, Yuna expresses awe at your skill with Mirages, stating that only Lusse Farna ever held such power. Lann notes that Lusse Farna was their mother, and Yuna informs you that Lusse Farna has been dead for over 100 years. She goes on to tell you that Lusse Farna came from another world to save Grymoire, and Reynn ponders over her being a Champion. According to legend, Lusse Farna fought against a wicked force known as the Demon Dyad, created the Ultima Gate, and through it summoned three Heralds. Turns out the Heralds stayed here in Grymoire, and reveals that one of them is now Brandelis, King of Bahamut. After saving Grymoire, the Heralds amassed a legion of Mirages and slowly took control of the land, building a powerful Federation. Nobody knows what happened to Lusse Farna, however, some say that summoning the Heralds cost her her life, while others say she lives as a prisoner in the Heralds' castle. Yuna tells you that if you wish to find Brandelis, you should ask around the Federation states, and informs you that Saronia just recently joined the Federation.

As you leave, the Plumed Knight swoops in and kills Yuna, only to discover that it was an illusion. Once you regain control, head north to the Ice Region.