—Edelgard, to the Immaculate One
To the End of a Dream is the final chapter of the Crimson Flower route in Fire Emblem: Three Houses. It is not possible to return to Garreg Mach Monastery during this chapter; the previous chapter and this one must be completed consecutively. This chapter consists solely of a mission, The Fight for Fhirdiad.
Lord Arundel meets the Adrestian Empire on the road to Fhirdiad, ostensibly to watch humanity take back Fódlan with his own two eyes. Afterward, Edelgard takes Byleth somewhere private to reveal that they and Rhea share a bloodline with the goddess, likely through Byleth's mother, and that they hold a special place in her heart for siding with her instead of Rhea.
The Black Eagle Strike Force arrives in Fhirdiad, and Edelgard offers the members of the Church of Seiros a chance to surrender. Meanwhile, Rhea orders Catherine to set fire to Fhirdiad, much to the latter's shock, and transforms into the Immaculate One. Edelgard rallies her friends and companions one last time, and the final showdown begins. The Adrestians engage Rhea, the Church of Seiros and remaining soldiers of the Holy Kingdom of Faerghus in a tumultuous battle, and emerge victorious.
Byleth and Edelgard together land the killing blow against the Immaculate One, but shortly afterward Byleth drops dead as the power of Sothis leaves them and Edelgard mourns over her professor's death. However, the Crest Stone implanted in their heart disintegrates and their heart begins to beat for the first time. Having united Fódlan, the Black Eagle Strike Force return to Garreg Mach Monastery and Edelgard begins her campaign to stop Arundel and Those Who Slither in the Dark.
Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
This is your final battle. If you've been saving stat boosters and Legendary Weapons for a rainy day, this is the time to bring them out. Make sure to replenish/equip all battalions before entering the battle, as well as all weapons.
Rhea has torched Fhirdiad, creating tons of fiery floors which halve movement of all units as well as damage them every turn; alongside stairs, for this reason, cavalry units are strongly discouraged. In addition due to the fact that fire drains and slows down units, bringing several units with healing items, Heal, Recover, Physic, and Fortify (Only available on Mercedes who you must have recruited pre-timeskip). Warp is also incredibly useful for warping units and saving time walking to places. In a similar vein, Flying Units are incredibly useful since they are immune to fire and can ignore the effect of stairs.
Your units are divided into sections, with 6 in the center, with the other 6 on the bottom right with a path that leads up to Catherine. Your first order of business will be to group off your units back into the center of the map, but before you that, you need to dispose of the Paladins and Snipers at the beginning. For the bottom right, you need to take out the Altered Golem that stands right in front of your unit. Their 1-4 range in conjunction with their minor Crest of Seiros, which grants them higher damage output, makes them formidable foes. What's worse, every three turns, Rhea will roar granting permanent stat boosts until +10, making them outright lethal. Luckily, most of them won't attack until provoked, so only the ones on the way to Rhea have to be dealt with.