Magic. How's it work?

Well, there's no Mana. However, by default, all magical powers require a skill check to succeed. If that check fails, or has mixed success, there are several bad things that can happen, including:

• A mishap, where the power happens, but not as you intend
• Blowback, where the power does what you want, but causes collateral damage, especially to you
• Strain, where the power happens as you wish, but at a cost to you

Strain is a situation where you have pushed yourself too hard, and subsequent magic use is going to be harder. The simplest idea is that Strain is a number, which is applied as a penalty to future magical skill rolls (until reduced through rest, healing, etc).

One idea is that you can "buy" better results by trading numeric amounts for side effects. For example, you roll a 17, but the DC was 25. You could choose from this "menu":

• Strain: any amount, 1 for 1
• Mishap: +5, but you also damage allies (or heal enemies)
• Mishap: +10, but DM decides targeting
• Blowback: +5, but you take the same damage
• Bowback: +10, but you and allies take the same damage
• etc

What is the DC? Doesn't the skill check set a DC for enemies to dodge/save/etc? Does their defense determine the Strain you take? Well, I mean, kind of. But there should also be a minimum DC for any power. Magic is dangerous, and bigger magic is more dangerous. Perhaps something like:

• Rank 1 power: DC 15
• Rank 2 power: DC 20
• Rank 3 power: DC 25
• Metamagic modifies further