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Terra and Zidane are ambushed by The Emperor and Sephiroth. The Emperor subdues Zidane in an electrical trap while Sephiroth overwhelms Terra. As the two make to finish them off, Kuja appears, accompanied by a Theme Music Power-Up of Dark Messenger, the Boss Remix of his Leitmotif. He frees Zidane, pushes Sephiroth back, heals Zidane and Terra, and the three stand together as they collectively enter Trance for the coming fight.
The game seemingly confirms that in Dissidia continuity, the Warrior of Light is a Composite Character of the four unnamed heroes from the original Final Fantasy. In other words, the Warrior completed that entire quest alone.
The cutscene before the final boss delivers in-story what the openings for the last two games teased — an all-out brawl between the characters. Highlights include:
Y'shtola protects Terra from Ultimecia and the Cloud of Darkness using one of her barriers. In response, Ultimecia and Cloud of Darkness combine their powers into a massive magical orb, only for it to get blasted into smithereens by a massive pillar of fire. Cut to Shantotto laughing with delight.
Vaan hurls a Pyroclasm fireball at Kefka, who casually dodges it...only for Tidus to jump up, spin and Jecht Shot it back into his face.
Exdeath teleports to avoid strikes from Onion Knight and Zidane, leading them into one of the Emperor's traps. Firion swings by to get the Emperor's attention, bailing them out.
Lightning and Sephiroth battle among falling debris, moving so fast they're practically blurs. When Sephiroth disarms Lightning, Noctis grabs her sword and uses it to Warp Strike to her and tosses it back to her.
Squall and Jecht have a short one-on-one fight, but Squall can't get a solid hit in... until he fakes Jecht out with one swing and pulls the trigger for a point blank shot. All that does is piss Jecht off, as he rushes out of the smoke and punches Squall so hard he flies back into an outcropping of rock that shatters from the force.
Golbez levitates the shattered rocks up as Bartz jumps up and throws a spear at him. Golbez just moves a hand aside to block it while using the other to send the rocks raining down on Squall.
Finally, Jecht takes on Bartz and Squall simultaneously, blocking every one of their strikes, while Cloud goes to face Golbez, slicing up the rocks.
Garland uses his BFS to bring a massive two-handed blow down against the Warrior of Light. He blocks it, his shield and sword glowing with light energy, and he sends Garland flying back with a Shining Wave.
The crown jewel happens towards the end just as everyone clashes. The heavens split open and Shinryu himself makes a grand appearance before laying waste to everything a la Bahamut on Eorzea. Everyone, ally and enemy, works together to shield against the blast, then press for the counterattack.
Spiritus and Materia combine their powers to entrap Shinryu, the magically-inclined characters pool their energy to blast the dragon in the chest, and the physical heroes slash the point with their blades, with the Warrior of Light using a Shining Wave to slice Shinryu in two.
Unfortunately, all that does is annoy the real Shinryu inside the golden apparition they just killed. Shinryu opens its eyes and sends everyone flying back in another wave of destruction.
For that matter, the Final Boss itself. The Warriors of both gods put aside their differences and work together to defeat Shinryu, the Greater-Scope Villain responsible for all these cycles of war in the first place.
It also bears something for the player. Shinryu is a notorious bonus boss in other games. Here, you get to take a fight straight to it and it is satisfying to beat him down dragon's maw to your fist.
So you defeat said Final Boss, the iconic Final FantasyBonus Boss that everyone knows, and watch as it collapses and falls into the void... only for it to rise up again bigger, darker, and even more pissed. Yes, unfortunately even Shinryu can go One-Winged Angel.