Author’s Note: This is a revised version of my article on Improvised Magic (May 13, 2011). All costs have been playtested and run through a spreadsheet to number-crunch every official spell into this system. Consider this the final version.
In many games with magical power, it isn’t that mages codify the supernatural so much as they enforce their will on the world and the supernatural provides the effect. As they practice, they become able to generate the same or similar effects over and over, so often that they “learn” a spell. Still, sometimes in a game with a limited number of known spells you yearn to have the ability to improvise something off the cuff, even if you know it won’t be as easy as casting a spell you already know.
So, I present the following thoughts for the generic AGE system. I am not sure whether this fits Dragon Age specifically, but I don’t think it matters for canon. If it works for your table, use it, or modify it. These ideas were inspired by Shadowrun and the discontinued Dragonlance Fifth Age game.
Improvised Spell System
When a mage improvises a spell, he decides on a clear effect and then uses the following charts to place it into game terms. With that final context decided, the player can cast his spell. Each component of the spell adds to the total Value of the improvised effect. This value is important in determining the TN and the mana cost.