Inside Sin

Inside Sin, known as just Sin in the menu (but called Inside Sin at the Monster Arena) is a location in Final Fantasy X, and the game's final dungeon. It is located inside Sin, a gargantuan whale-like beast that threatens all life on Spira.

Story
After Summoner Yuna has abandoned her pilgrimage and decided not to let Yunalesca turn one of her guardians into the Final Aeon to defeat Sin and battled her, the party looks into new ways of tackling the beast. They decide to use the party's airship Fahrenheit to battle Sin in a way no one has ever attempted before.

When they manage to penetrate Sin's armor, the party makes their way inside Sin, and toward the center, where Yu Yevon rests. On their way deeper into the beast, the party face Seymour Guado who has been absorbed by Sin, for one final battle. At the center of the gargantuan beast, in Dream's End, the party finally meets Jecht who transforms into Braska's Final Aeon.

Areas
Most of the areas are based on the real life models of early Zanarkand except that it is a distraught projection of it.
 * Sea of Sorrow - A haunting, red-tinged sea winding through the landscape, it is traversed through a series of waterfalls and watering blue platforms. There are symbols of Yevon surrounding this place.
 * Garden of Pain - A series of steps leading to a platform, it is where the battle with Seymour Omnis takes place. Once he has been defeated, this area is bypassed on all subsequent trips through Sin. However, the staves will remain.
 * City of Dying Dreams - A maze of moving platforms and teleport glyphs which must be navigated. Various side paths lead to treasure chests containing weapons and items.
 * Tower of The Dead - As the player approaches what appears to be an exit to the city, a large metal tower descends and blocks their path. Examining a glyph on the side of the tower will teleport them past the Point of No Return and to the next area.
 * The Nucleus - A surreal environment of crystalline floors and walls; the goal is to touch the colored crystal orbs which appear in the area before they disappear. The camera angle is always moving, adding to the disorientation. An item is awarded each time a crystal is touched, but touching the icicles which sprout from the floor will cause a battle to begin. After touching ten crystals the player is transported to the next area. The area is briefly seen after Operation Mi'ihen's failure.
 * Dream's End - A ruined version of Zanarkand's blitzball stadium with a platform leading to the center of the arena. Here the party meets Jecht and fights Braska's Final Aeon. When it has been defeated, Yu Yevon reveals himself, searching for a new aeon to possess.

Enemy formations

 * Sinspawn Genais, Sin (Boss)
 * Adamantoise
 * Ahriman x3
 * Ahriman, Exoray x2
 * Barbatos
 * Behemoth King
 * Demonolith
 * Demonolith x3
 * Gemini (Mace), Gemini (Sword)
 * Great Malboro
 * Land Worm
 * Varuna
 * Wraith
 * Seymour Omnis, Mortiphasm x4 (Boss)
 * Braska's Final Aeon, Yu Pagoda (Boss)
 * Valefor, Yu Pagoda (Boss)
 * Ifrit, Yu Pagoda (Boss)
 * Ixion, Yu Pagoda (Boss)
 * Shiva, Yu Pagoda (Boss)
 * Bahamut, Yu Pagoda (Boss)
 * Anima, Yu Pagoda (Boss)
 * Yojimbo, Yu Pagoda (Boss)
 * Magus Sisters, Yu Pagoda (Boss)
 * Yu Yevon, Yu Pagoda (Boss)

Dissidia Final Fantasy
The Dream's End area is the representative arena of Final Fantasy X in Dissidia. Unlike most arenas in Dissidia, Dream's End is only used for the two fights against Jecht: one in Tidus's storyline and the other in Shade Impulse. No other bosses are fought at Dream's End, though a handful of cutscenes occur here, including Firion and the Onion Knight's respective conversations with Cosmos.

The arena consists of a ruined Blitzball stadium surrounding a circular platform over a field of lava, which is treated as a Banish Trap. A massive replica of Jecht's sword appears on the central platform, much like after defeating Braska's Final Aeon in the original game. Long grind bars run along the outer edge of the arena, connecting the central platform to the ruined "stands" on the opposite side. In the Ω version of the stage, every time a character guards, dodges, or lands an attack, the Bravery pool increases, and when damage is dealt it increases by an amount equal to the damage dealt.

The Moonflow is the battlegen item obtained through stage destruction in this area.

Dissidia 012 Final Fantasy
Dream's End returns as an arena in Dissidia 012. In the 12th cycle, Yuna faces off against the Emperor at this stage after he struck down Tidus.

Theatrhythm Final Fantasy
Dream's End is the battle background for "Fight With Seymour" and "Otherworld". When playing "Fight With Seymour" during a Dark Note, the battle background featured is the Garden of Pain.

Musical Themes
The theme that plays Within Sin is "Twilight".

Trivia

 * The final area of Sin, Dream's End, is actually visible in the opening moments of the game. When Tidus is first absorbed into Sin, he is teleported to Dream's End, where a vision of Jecht stands in the central platform. After Tidus "swims" toward the platform, a cutscene occurs and Tidus is transported to the submerged ruins.
 * Another section, the Nucleus, is seen briefly in the aftermath of the disastrous Operation Mi'ihen.
 * The name of the place where the party fights the aeons and Yu Yevon was never revealed.

Dentro Sin