I hope this letter has gotten to you in good circumstances, though the content may change that.
The strange mark that has appeared on your body is calling. A calling to arms. You, like many others in our lands, have been gifted with magic. However, this particular magic means you have been chosen by the magical forces that be to join the age-old tradition of dragon riding. There is a dragon waiting for you on the island of Medeis, and you will do well not to keep it waiting. If you do not come to the dragon, the dragon may end up coming to you in the future - and you will be responsible for the outcome of that situation.
Now, I hope I haven't intimidated you too much, my friend. Pack a bag of personal belongings and make your way to the port of Porro in Fides - you will find in the envelope the appropriate amount of money to acquire transport from your current location. Do not tell those around you the exact purpose of your journey. If they must know something, you tell them you have been summoned by a higher force and you must answer. Tell them you will return in due time, just hope they accept your fate. If they do not accept it, steal away in the night if you have to. Just get here. I expect you in Porro by September 1st, go to the Portside Inn and wait to be approached.