When casting a spell, a wizard has to constantly concentrate on it for a specific period of time (taking Concentrate maneuvers every turn). Their energy reserves are limited by any FP currently having an optional Powerstone. In other words, except for additional decisions, if the spell costs 16 energy points and the magician only has 14 FPs of his own, they cannot cast it without a stone of power large enough: they cannot stop in the middle of the cast, rest to recover the spent energy, and then continue.
However, things are a bit more complicated with Ceremonial magic (M12, B238), when the magician can get one or more assistants to contribute more energy to the launch, at the cost of ten times more launch time and several other penalties. There is a particular note that caught my attention:
- Once the spell is cast, participants can continue to provide energy to maintain the spell. The composition of the group can change, as long as the ritual continues without interruptions. Therefore, ceremonial magic allows you to maintain a spell indefinitely.
It is not clear at all if the composition of the group can change during The casting, however. That would do It makes sense that he can, after all, continue the ritual uninterruptedly and the "pitcher" – the group as a whole – constantly concentrates on him. Surely, some of the participants should be able to leave for a while, possibly replaced, rest and return, if the ritual is designed so that such permits are agreed in advance and are not the result of any accident or interruption.
The Enchantment rules ("Slow and safe enchantment" – M18, B481) state that, at least with this specific form of ceremonial magic, participants can regularly interrupt the launch to rest in the middle. Is could have been which implies that the interruptible launch has a high price of extended time and required resources, since the other form of enchantment ("Fast and dirty") is much cheaper, but has no decisions about interruptions; however, in this case, the group does not maintain constant concentration, the consequences of possible interruptions do not ruin the entire spell (only one working day), there are other benefits on QoD enchantment (without penalty of skill for the number of participants), as well as suggestions for GMs who wish to restrict even more the QoD enchantments In other words, this is not a definitive indicator.
On top of that, there is a Power Cone spell described in Thaumatology (page 52) that allows you to do things very similar to an interruptible casting, but it still has many significant differences.
On the other hand, an interruptible ceremonial launch could have enough balance problems. For example, a distant cult in a forgotten temple in a haven of the world could cast a Fire Rain spell on all world over the course of several years without anyone noticing … and then unleashing everything at once! Then again:
- It only takes a random foray (or a rock fall ™ plus a failed Wil launch) to ruin years of preparation, so it clearly cannot be done without the permission of a GM, implicit or explicit;
- "Cone of Power" also allows you to do that, and it is much more dangerous: after all, the energy collected can be used prematurely to repel potential attackers. However, this spell it is in an official guide;
- It is really a hook for an adventure!
The Cult of Doom has been preparing a terrifying and gigantic spell to unleash the world, which our fortune tellers have learned a moment ago, and now someone must sneak into their highly protected lair to interrupt the pitchers! Bonus points for cinematic arrival just in time to interrupt the cult just when they are about to finish casting.
Is there an official documented decision on whether wizards can change the composition of the group during the Ceremonial Release without interrupting the spell? And if not, what balance considerations should GM consider when making its own decision?