GURPSよろず

　You have access to the Mass Magic rules (Thaumatology, p. 51) in a world where these aren’t generally available.

　Ceremonial Magic (p. B238) enables wizards to draw on multiple supporters for energy.
Below are some optional rules that expand on this technique.
These methods might be widely used – but they could instead be secrets known only to a few specialists, requiring a perk or an Unusual Background.
They especially suit megalomaniacal wizards with lots of political power.

　Enough propaganda (a successful Propaganda roll at -3, using media that can reach at least a city-sized community) or a sufficiently mesmerizing oration (a critical success on Public Speaking, or possibly on Religious Ritual for clerical magic) can increase the amount of energy that spectators can contribute by creating a quasi-religious atmosphere.
For each week of propaganda blitzing beforehand, or each point by which the Public Speaking or Religious Ritual roll succeeds, spectators can contribute an additional FP to the ceremonial casting, to a maximum of half their FP.

　Mass gatherings might serve as sources of extensive ritual power, utilized by adepts on the dais or scattered anonymously through the crowd at key points.
The casters must organize or design the assembly in question, inserting their spells, often subtly, into the activities.
This may require one or more rolls against Thaumatology (or Theology, for clerical workings).
It might be possible to hijack a preplanned event for magical ends with successful rolls against Politics, Psychology, Ritual Magic, and/or Artist (Scene Design).

　Each wizard involved in the spell – who must know it at level 15 or better – can either personally contribute energy to the spell or coordinate up to 10,000 spectators in chanting, marching, or other ritual actions that will generate one energy point per 100 participants, up to 100 points total.
Such coordination requires a successful Ritual Magic roll (use Religious Ritual for clerical magic).
Failure grants no energy, while critical failure drains 100 points from the total.
Fanatical spectators (see above) grant double energy.
“Unbelievers” still lower the total, blocking one point per 20 actively hostile onlookers or participants.