Call of Colossus

[Ryastar]

Can anyone run through how to change what troops you get from these things, and for that matter what troops you can get. I'm trying to get red dragons, but I've had no luck so far. I've tried the scroll on every island I have access to, which basically meant a quicksave before travelling, travelling, using the scroll, reloading, and trying again with a different island. Am I doing it right and just having abysmal luck, or is there some other way to manipulate the outcome?