The Santa Claus Principle

The Santa Claus Principle is a general metaphysical law that governs the nature of divine interventions.
As the title suggests it's based upon the principle that mortals tend to have preconceived ideas about what their gods and devils look like. This imagery is often recorded in manuscripts and depicted in iconography and idolatry. Thus if such a divine being were to manifest before them most mortals would know exactly who and what it represented.
'The Santa Claus Principle' is based on the idea that if one were to gather a few hundred school-age children in a room and ask them all to draw a picture of Santa Claus it's almost certain that all of them would produce a very similar image, even though none of them has ever seen Santa Claus in person.
The point being that mortals are conditioned to expect mythical beings to conform to the images described by their culture.

The Santa Claus Principle in Practice

So, how does the Santa Claus Principle work in WFRP Fragile Alliances?

• Firstly it is assumed that the imagery of a divine being published in the 'Fluff' is the accepted imagery promoted in that culture. Thus just as children know exactly what Santa Claus looks like, we assume that most mortals in our world know what their specific gods look like. They have seen depictions of them in manuscripts, in paintings, on stained glass windows in temples and as statues and idols.
• Secondly given that divine intervention is a combination of warp energy stimulated by psychic emanation it is reasonable to assume that the manifestation of the divine being generated by the mortal psyche would take the shape and form expected by the mortal, or mortals, responsible for causing it to appear. So, if a pack of Skaven were to trigger a divine intervention of their god, it would almost certainly take the form of a large horned rat because that's what they all expect their god to look like.
• Finally, it follows that there is a perverse logic at work in this process, in that, what mortals cause to become manifest is exactly what they are conditioned to believe exists. But in reality, it is that conditioning which is the primary cause of the nature of the manifestation. So, if one takes as an example the portents of doom propagated by The Cult of Sigmar and their warnings about the horrors of the dark gods of Chaos. It is ironic that it is probably these very warnings that cause the manifestations of the dark gods to take such horrific forms. In the same way, as if we were able to manifest the Devil he would almost certainly be a big red evil looking monster with horns.

Thus when handling a divine intervention the GM will take into account the expectations of the mortal or group of mortals who are triggering the event and tailor the imagery of the manifestation to match those expectations.
It can even mean that the same manifestation could appear quite differently to mortals from different cultures. Though generally divine interventions are culturally specific and so will appear in the image expected by the mortals triggering the event. So, if a pack of Skaven summon The Horned Rat then they will see what they expect The Horned Rat to look like, and any elves, humans, dwarves or mortals from other cultures will witness the same manifestation as a projection of the Skaven psyche.

Bending the Rule

Whilst the Santa Claus Principle remains the foundation of all Divine Interventions the GM will occasionally bend the rules slightly.
For example, the 'Fluff' often describes multiple variations in the imagery associated with a god. Thus, for example, Ulric is often depicted as a white Wolf, whilst Taal is often depicted as a white stag. The GM may therefore quite legitimately choose to describe the manifestation triggered by divine intervention in any accepted form described in the 'Fluff'
Likewise, particularly when the divine intervention is linked to an individual character rather than a group the GM may decide to include manifestations that will be readily recognised by that individual even though they are not universally associated with a divine being. For example, a dwarf might well find itself being visited by one of its family ancestors.
This allows for a certain degree of variation by the GM to personalise a divine intervention experience.
The following is an actual example of a divine intervention from my last game, in which, Bertoldo the thief receives a warning from Ranald the god of thieves appearing in the form of a black cat.

Bertoldo glances past the cordon to where the pile of wool bales unloaded earlier are still stacked on the quay awaiting collection. A group of feral cats have gathered around the bales inspecting them carefully for signs of rodent occupation. All that is except one particularly skinny black one which has no interest in hunting and is being ignored by the rest of the group. Instead, it stares accusingly at Bertoldo through the legs of the militiamen fixing him with its two green eyes. A breeze whips over the quayside from the river making Bertoldo shudder and a thin voice whispers in Bertoldo's ear "Betray me Bertoldo and I shall drop you like a baker drops a hot muffin." Bertoldo looks quickly over his shoulder expecting to find someone standing behind him but there is nobody there and when he turns back the black cat has gone.

'Mortals need to believe in things that aren't true, for how else can they become.'

Terry Pratchett.

'Belief is one of the most powerful organic forces in the multiverse. It may not be able to move mountains, exactly. But it can create someone who can.'

Terry Pratchett

People get exactly the wrong idea about belief. They think it works back to front. They think the sequence is, first object, then belief. In fact, it works the other way. ― Terry Pratchett, Reaper Man

Mortals are conditioned to expect mythical beings to conform to the images described by their culture.

The Santa Claus Principle