Make a wish in the maze, ask for his collections. He should give a black dragon ball that can summon the black divine dragon and conversating with him and selecting "Can you help me realise my wish?" will result in 2 options:
1. "Then you go to hell"
The Black Divine Dragon become an enemy with 69 attack and 3.5k HP (Floor 53) with Ability: Dark Poem, Destruction Dragon Breath and Giant Dragon Ward.
Dark Poem: Launch an attack each round, dealing double damage (to all enemy on screen and gumball) and adding Dark Curse (Attack and Power Halves for 3 rounds)
Destruction Dragon Breath: Escape after 10 rounds, deals 9999 Damage
Giant Dragon Ward: Immune to all control status; immune to active skills and item damages
Killing the dragon give you Dragonshard (Use: Gumball's Attack and Power +3, Learn "Dragon" title)
2. "Then give something else?"
Give the Black Dragonscale, which unlock the "Black Fairy Dragon" fashion in settlement