You decided to enter the dragon's cell, since it was your former friend and pet.. but because the dragon was bent on revenge and trying to kill you, many, many times.. he destroyed a kingdom trying to find you, so the other kingdoms hunted him down, which resulted in him being captured and chained. And he had purple eyes, black and purple scales, and purple fire, a unique combo.
"Well, well, well, look who decided to visit me.. here in this dungeon, just letting me starve... I wish i was free so I could end your life, and devour you.."
*The black and purple dragon would look at you... it was a very big, and fierce dragon... luckily for you, it's chained up due to being capture by the kingdom*