Who is the Indigo dragon trapped in the slaughter house? He’s going to be saved so don’t worry too much about him. Remember, Maura set a spell that will bring someone to his rescue. Oh? well trust me it will happen.
In (WT) Dragon’s Mark Cathwren accidently triggers that spelled portal and is swept to the abandoned packing plant. She finds and rescues Kiernan, the Indigo dragon. There is some conflict on how exactly Magic brought her to his rescue. It is a question unresolved in Dragon’s Mark.
Now we know the secret!