Those mages who succeeded in chapter 5 of the campaign, which is "Wizard's tower", will learn the art of turning their cards into the Magic sources.

Magic source is deprived of its power but retains full potential of the card it is made of. This potential will be used if one improves a card by absorbing the source into it.
It is known that different cards may possess different power on the same level. This is why sometimes it is more practical to improve one of your weak cards and move it to the combat deck if it gives more power on the higher levels.

The sources can be very useful if you want to move a card out of the combat deck. All you need to do is just turn that card into the magic source which will get to the bag right away provided you have got at least one weak card to replace it in the deck. Your combat deck cannot be smaller than 9 cards.

Magic sources are given away as prizes in the Tournament, in the Guild Wars, for improving guild card (each 5th level) and also in the Chest Shop.