Past the previously-sparkling door is a lava cave. Every fight in the lava cave is with some breed of Dragon. Check your subscreen, and it'll tell you explicitly: this is Shinryu's lair.
So you can guess what's in the last chest in the area.
Neo Shinryu is even stronger than regular Shinryu! Hooray. And despite appearances, there are four targets in this fight. Three of them do nothing, but they're utterly invincible and absorb hits from multitarget attacks (like !Rapid Fire... sensing a pattern?). The actual Neo Shinryu toggles invincibility on and off, too, so statistically 50% of attacks against the right target still miss.
Neo Shinryu attacks with all the usual stuff, from Tidal Wave to White Hole to Earthquake and... erm, Almagest.
Oh, and he always starts the battle with Mighty Guard, because nothing makes fights with superbosses better than when they Haste, Protect, and Shell themselves.
Despite being a native Level 97, Neo Shinryu is somewhat vulnerable to Dark Spark, and once that's set, he can be aged to slightly decrease his damage output as the fight drags on.
And boy does the fight drag on. Everyone has Coral Rings, Ice Shields (except Faris, who has a two-handed harp as shown), and float status from Mighty Guard, so several of Neo Shinryu's attacks are absorbed or nulled. Ice Storm, Tidal Wave, Earthquake, and Blaze are simply ignored by the party. The Ribbons everyone is wearing null his Curse, Doom, Poison Breath, White Hole, and Zombie Powder procs. So Neo Shinryu is wasting as many turns trying to hit us as we are trying to hit him! looooool
Anyway, that harp Faris is equipped with is the Apollo Harp, which does octuple damage to dragon-type monsters. So whenever Faris lands a hit, it's 9999 damage. Lenna spends most of the fight healing everyone, while Bartz keeps everyone buffed up with !Mixtures. Krile... spends most of the fight hiding.
Because after a while, Neo Shinryu will cast Gigaflare...
...followed immediately by Meteor. I wasn't sure how fatal that combination was going to be, and god damned if I'm going to lose the fight to Neo Shinryu's death rattle.
Oh yeah, that combination is his dying counterattack. So I won. It took almost an hour.
The reward is the best weapon in the game.