So, I've been doing some more thinking and I now have a pretty complete retooling of Evocation to Fate Core, making as few changes as possible. I'm using the idea proposed by Leonard Balsera earlier in this thread.
Evocation can be used to Attack, Overcome, Create an Advantage and Counterspell. Overcome is just using an evocation to overcome some difficulty in the scene or achieve a fixed result and is resisted actively or passively as normal.
So the only one that really needs explaining is Create an Advantage as it sticks around.
Under these rules Evocation can be used to establish an effect of indefinite duration as long as active concentration is maintained. Fast, simple actions (free actions in the rules) won't disrupt concentration but doing anything that requires an action is difficult without losing concentration on the sustained evocation. To represent this the difficulty of taking any other action is increased by 2. As an addition to any other consequences of failing or tying the action roll, the sustained effect is ended.
Note: I was thinking about requiring a Discipline roll at the same difficulty as the original evocation to take any other actions but decided that it would be best not to add a second roll.
Rather than duration, extra shifts of power are used in order to gain additional free invocations. The first is gained with 1 shift of power and another is gained every for every two shifts. You would therefore gain free invocations at 1, 3, 5 and 7 shifts of power.
Once the evocation is in effect it can be used exactly like any other scene aspect. Free invocations can be stacked to provide multiple +2 bonuses or rerolls at any given time.
There are some specific things to bear in mind when this system is used to create shields.
The user can, if they choose, gain personal armour at a cost of two shifts per point. As per the original rules this is not depleted if they take an injury. Shifts used to create armour in this way do not provide free invocations.
Extra shifts may be used as per the DFRPG rules to spread the shield over a wider area. Again, these do not count towards free invocations.
When using a shield to defend against incoming attacks, the player can either use free invocations (and/or a single Fate point invocation) to make a passive obstacle of 2 per point spent per the Invoking an Aspect as an Obstacle rule (possibly costing the enemy a turn to blast through the shields - if successful - when invoked before the enemy's turn) or make an active defence roll against the attack using Discipline rather than Athletics. Again, free invocations and/or a Fate point can be used to increase the defence.
Only one Evocation can be sustained in this way at any given time.
What do you guys think?
