The Demon Seal has always been a double-edged blade in the Tales series- Always offering double exp. points, but always with a catch. While usually this meant disabling your special attacks and bringing your defense down to 0, this time around the programmers decided on something different- You get double exp., but it throws random status ailments on you. This could be crippling, but if you equip the Krona Symbol (which prevents all status abnormalities), you're COMPLETELY immune to the Demon Seal's curse, and free to reap your doubled exp. rewards! I won't tell you WHERE to get the Demon Seal or Krona Symbol; there should be at least some challenge left. But I will say that you can't get the Demon Seal until VERY near the end of the game...