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 . This is the only storyline location that must be manually filled in by the player by traversing the area , highlighting its unknown nature . Story . The final area of inside Sin , Dream ' s End , is briefly glimpsed when Tidus is first absorbed into Sin from Dream Zanarkand . He is teleported to Dream ' s End , where a vision of Jecht stands on the central platform . After Tidus " swims " toward the platform he is transported to the submerged ruins . Another section , the Nucleus , is seen in the aftermath of the disastrous Operation Mi ' ihen , where the spectral visions of the Crusaders are seen wandering around the more corporeal form of Bahamut ' s fayth . 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 , the party looks into new ways of tackling the beast . They decide to use the party ' s airship , " Fahrenheit " , to fight Sin in a way no one has ever attempted before . When they 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 faces Seymour Guado , who has been absorbed by Sin , for a final battle after which Yuna sends him at last . At the center of the gargantuan beast , in Dream ' s End , the party meets Jecht who transforms into Braska ' s Final Aeon . After being vanquished , Yu Yevon ' s spirit seeks a new host and possesses Yuna ' s aeons one by one . The party defeats the aeons until Yu Yevon has no more host bodies to possess and then defeats Yu Yevon himself . Upon Yu Yevon ' s death he stops summoning Sin and it explodes above Bevelle , while the party escapes on their airship . Locations . Most of the areas are based on the ancient Zanarkand except that it is a distraught projection of it . Items . In the City of Dying Dreams area , the player will find a glyph on the right wall . The first glyph tells the player to kill ten individual fiends . After doing so , return to the glyph and touch it to reveal another . The player must kill ten more to open the next door and then 15 to open the final one . A quick way to achieve this is to battle a party of three enemies ( like Exoray ' s group ) . After the player has opened all three doors , they will find a chest with a Lv . 4 Key Sphere . In The Nucleus area , small floating egg - like crystals appear alongside spike crystals popping out of the ground . The player must touch the floating crystals and avoid the spike crystals . The egg crystals disappear after a short time , so grab them quickly when they appear . Each one will give an item in a random order : Other appearances . " Dissidia Final Fantasy " . The Dream ' s End area is the representative arena of " Final Fantasy X " . Unlike most arenas , 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 the ruined Zanarkand 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 " Final Fantasy X " . 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 . In the 12th cycle , Yuna faces off against the Emperor here after he struck down Tidus . " Theatrhythm Final Fantasy " . Dream ' s End appears as the BMS for " Fight With Seymour " and " Otherworld " . When playing " Fight With Seymour " during a Dark Note , the BMS featured is the Garden of Pain . " Theatrhythm Final Fantasy Curtain Call " . Dream ' s End and the Garden of Pain return as the BMS for " Otherworld " , " A Contest of Aeons " , and " Fight With Seymour " respectively . Musical themes . The theme that plays inside Sin is " Twilight " . " The Unsent Laugh " plays when the party discovers Seymour .