The final boss can drop key items that can let you Class Change to any of the initial Classes (the ones that you can't already unlock by getting max affinity with your allies).
The 15th "dungeon" is an optional difficulty spike for any other dungeon (I'll call it Endgame+) after clearing the 14th dungeon (which ends the main "story").
>enemies are jacked to Level 100-250
>all equips become 6* when you return from an Endgame+ dungeon (basically the items with [11] or [12] rarity)
>clearing the 14th dungeon on Endgame+ adds ANOTHER difficulty spike each time (Endgame++, etc.)
>at some point the enemies will oneshot you even with max Counter stats because there's a hidden cap on Counter % and Wave of Nothingness ignores counters (haven't tested max Evasion yet)
Endgame+ is literally for the masochists who love to grind, there's no new scenes or ending.
My Endgame+ team was Iris/Clarica/Victoria, with my kit as Berserker (+20% Counter)/Lance Knight (has Taunt and Random Thrust). I also had 107% Magic Reflection and 77% Counter Rate. I'll think about Endgame++ if a Counter Rate +30% or +50% skill exists.
My boss skills:
Ailments Immunity
Counter +10%
Evasion +20%
First Aid
Lifesteal
Max HP +30%
M.Reflect +10%
M.Reflect +50%(!)
Physical DMG -10%
Physical DMG -50%(!)
Sidenote: You want to keep EXP Gain +50/100% until you hit Lv99, then you can toss them for anything else since you don't need EXP anymore. Item Acquisition +100% should be kept until you're done with the gear grind.