How does Spiritual Weapon interact with Illusions?

Question

There were two Silent Images mimicking a fight between a seemingly helpless woman and a harpy at quite some distance. The PCs (good souls that they are) decided to save the woman, and as soon as able the Oracle cast Spiritual Weapon and set it on the harpy.

In Pathfinder, Spiritual Weapon has no stipulations regarding how it interacts with illusions, although it is laid out pretty clearly otherwise. The spell states:

A weapon made of force appears and attacks foes at a distance... It strikes the opponent you designate, starting with one attack in the round the spell is cast and continuing each round thereafter on your turn... It strikes as a spell, not as a weapon, so for example, it can damage creatures that have damage reduction. As a force effect, it can strike incorporeal creatures without the reduction in damage associated with incorporeality. The weapon always strikes from your direction. It does not get a flanking bonus or help a combatant get one. Your feats or combat actions do not affect the weapon...

For the illusory side of this interaction, the magic entry for illusion states [emphasis my own]:

Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.

Would Spiritual Weapon count as "interacting with [the illusion] in some fashion"?

On the spot, as the DM setting a particularly fun trap, I didn't want to give it away quite yet so I did not allow a will save (which would definitely have given it away) but instead a perception check. Was this correct/is it truly DM interpretation?

Answer 1

Spiritual Weapon interacts with illusions normally

This is clarified in Ultimate Intrigue:

Disbelief and Interaction: All three of the subschools above tend to have saving throw lines that say “Will disbelief,” but they differ in how those saving throws apply. Phantasms directly assail a creature’s mind, so the creature automatically and immediately receives a saving throw to disbelieve a phantasm. Figments and glamers, however, have the more difficult-to-adjudicate rule that creatures receive a saving throw to disbelieve only if they “interact” with the illusion.

But what does it mean to interact with an illusion? It can’t just mean looking at the illusion, as otherwise there would be no need to make the distinction, but drawing the line can be a bit tricky. Fortunately, the rules can help to define that difference. A creature that spends a move action to carefully study an illusion receives a Will saving throw to disbelieve that illusion, so that is a good benchmark from which to work.

Using that as a basis, interacting generally means spending a move action, standard action, or greater on a character’s part. For example, if there were a major image of an ogre, a character who tried to attack the ogre would receive a saving throw to disbelieve, as would a character who spent 1 minute attempting a Diplomacy check on the ogre. A character who just traded witty banter with the ogre as a free action would not, nor would a character who simply cast spells on herself or her allies and never directly confronted the illusory ogre. For a glamer, interacting generally works the same as for a figment, except that the interaction must be limited to something the glamer affects. For instance, grabbing a creature’s ear would be an interaction for a human using disguise self to appear as an elf, but not for someone using a glamer to change his hair color. Similarly, visually studying someone would not grant a save against a glamer that purely changed her voice.

It takes a move action to move the spiritual weapon to attack a new target, and casting the spell on a target takes a standard action, so the requirement is fulfilled here. So, if you spent some time to interact with the illusion, it counts as an interaction and you are allowed a save.