Get Away From Me!
In a couple of places in the book, Harry blasts a foe with an evocation that catapults them away from him, giving him time to run away or deal with something else.

Now, as with any evocation, there are lots of different ways to apply the various elements to produce the effect; air and spirit are the most obvious, but a mystic wave of water or a blast of expanding air from a sudden heat source or the gravity of the target shifting 90 degrees would also all work. For our purposes, let’s go with spirit, in the guise of pure force slamming into the target and sending it flying.

Once we’ve chosen the effect, we have to pick what mechanics we’re going to use to model it. For this effect, it’s a little tough, because there are no rules for using evocation for movement – Harry points out that movement via evocation would be a wild, uncontrolled, unsafe thing. Because we’re applying this to an enemy, though, we don’t mind those problems so much.

The obvious option is to model it as a maneuver, applying the Aspect Knocked Sprawling or something like that*. If the point of the spell is to move the target one or more zones away from you while applying the Aspect, I’d say that it would take extra power to do that. And there we have the mechanic.

Next, we need to determine power requirements. Assuming a standard mortal as a target, that’s going to take 3 shifts of power to apply the maneuver. However, if we also want to move the target one or more zones away, that’ll take a little more power – I’d say 1 per zone, plus another 1 for every level of barrier between zones that the target needs to move through. So, Harry, with a Superb (+5) Conviction, can call up 5 shifts of power for only a 1-stress hit. That’s enough to toss a normal target 2 zones away if there’s nothing in the way. If there’s a barrier:1 fence in the way, the target’s only going 1 zone away. And a barrier:2 wall means the target’s not going anywhere. Let’s keep it at 5 shifts for our calculations.

So, 5 shifts is easy to call up, with only a 1-stress price tag. But it’s still more power than Harry can readily control with his Good (+3) Discipline and his focus item (his staff). He’ll need to roll at least a +1 on the dice to keep from having to worry about backlash and fallout, which he should be able to do 38.3% of the time. Assuming success, the target should probably get a chance to oppose with either an Athletics roll (to dodge) or Might roll (to tough it out and not be moved).

Final version of the spell looks like this:

Knockback
Type: Spirit (force) evocation, offensive maneuver

Power: Varies; typical is 5 shifts – 3 for effect plus 2 for distance

Control: Roll Discipline plus appropriate specializations and focus items.

Duration: One action

Opposed by: Target’s Athletics or Might

Effect: If the spell hits, the target has Knocked Sprawling temporary Aspect applied, and is thrown one zone away from the caster for every extra extra shift of power. Barriers between zones reduce the distance by their barrier value: i.e., the shifts of power must overcome the barrier value to move the target through the barrier.

Notes: GMs may wish to apply some damage to a target passing through – or failing to pass through – a solid barrier.

https://docs.google.com/document/d/1NFN0p2aZ24ttiS8FxWWGBrwNjpwEDZY5nN4enZgAOHg/edit?hl=en_US&pli=1#

If the PCs attack the Tower and get swarmed, the ridiculous increase in cultist opposition... is that something they can handle?  Is it something that's supposed to tell them that this was a bad idea?