When casting a spell, a mage has to constantly concentrate on it for a specific time period (taking Concentrate maneuveres each turn). Their energy reserves are limited by whatever FP they have at the moment plus an optional Powerstone. In other words, barring additional rulings, if the spell costs 16 energy points and the mage only has 14 FP of his own, they cannot cast it at all without a sufficiently large powerstone - they can't stop in a middle of the casting, rest to recover the spent energy, and then continue.
However, things are slightly more complicated with Ceremonial magic (M12, B238), when the mage can get one or several assistants for contributing more energy to the casting, at the cost of tenfold increased casting time and several other penalties. There is one particular note that grabbed my attention:
- Once the spell is cast, the participants can continue to provide energy to maintain the spell. The composition of the group may change, as long as the ritual continues uninterrupted. Thus, ceremonial magic lets you maintain a spell indefinitely.
It's not clear at all whether the composition of the group can change during the casting, though. It would make sense that it can - after all, the ritual continues uninterrupted and the "caster" - group as a whole - keeps constantly concentrating on it. Surely, some of the participants should be able to leave for a while - possibly getting replaced - rest and return, if the ritual is designed so that such leaves are agreed upon in advance and are not a result of any accidents or interruptions?
The rulings on Enchantment ("Slow and Sure Enchantment" - M18, B481) state that, at least with this specific form of ceremonial magic, the participants can regularly interrupt the casting to rest in between. This could have been implying that interruptuble casting has a heavy price of extended time and resources required - as the other form of enchantment ("Quick and Dirty") is much cheaper, but has no rulings on interruptions - however in this case, the group does not maintain constant concentration, the consequences for possible interruptions do not ruin the whole spell (only a day's work), there are other benefits over QoD enchanting (no skill penalty for the number of participants), as well as suggestions for GMs who wish to further restrict QoD enchantments. In other words, this is not a definitive indicator.
In addition to that, there's a Cone of Power spell described in Thaumatology (page 52) which allows to do things very similar to an interruptible casting, but still has many significant differences.
On the other hand, an interruptible ceremonial casting could have quite a few balance issues. For instance, a distant cult in some forgotten temple in a backwater of the world could be casting a Rain of Fire spell over the entire world over the course of several years without anyone noticing... and then unleash it all at once! Then again:
- It takes just one random raid (or a falling rock™ plus a failed Wil roll) to ruin years of preparation - and so it clearly cannot be done without a GM's permission, implicit or explicit;
- "Cone of Power" allows to do that, too, and is way more dangerous - after all, the collected energy can be used prematurely to repel any potential attackers - yet this spell is in an official guidebook;
- It's actually quite a hook for an adventure!
The Cult of Doom has been preparing a terrifying, humongous spell to unleash upon the world, of which our diviners have learnt just now, and now someone must sneak into their highly protected lair to interrupt the casters! Bonus points for cinematic arrival just in the nick of time to interrupt the cult just as they are about to finish the casting.
Is there an official, documented ruling on whether the mages can change group composition during Ceremonial Casting without interrupting the spell? And if not, what balancing considerations the GM should keep in mind when making his own ruling?