In Pantheon mode, you choose to side with either Light or Darkness, and that choice is permanent for that run of the tournament. You can play any demigod you want in skirmish or custom games, but in Pantheon you're limited to the good ones or the evil ones.

If you don't like your current choice, fear not, as it will be reset this coming week.

I havent played the game yet but considering you can only select light Demigods, i will say that you are playing pantheon and you decided to go with the light side wich only lets you chose demigods of the light side.

thanks for the replys guys hehe silly me i realize how silly of a mistake that was