This was originally published in April of 2014. Hack & Slash compendium III is available in print in from Lulu and in PDF from DTRPG, and contains this series of articles on illusions, as well as many other class ideas.
A good rule of thumb is that a figment effect has its subdual damage limited by caster level.
Phantasmal Force (or Silent Image) might be limited to 1d4 subdual damage per two caster levels.
Improved Phantasmal Force (Or Minor Image) Might be limited to 1d6 subdual damage per two caster levels.
Spectral Force (or Major Image) might be limited to 1d8 subdual damage per two caster levels, or a minor or temporary status effect.
Having a Spectral Force of a Medusa would cause a save versus spell because seeing a Medusa is interacting with it. But if that saving throw failed, a viewer would save versus the gaze and believe themselves turning to stone perhaps actually acting as if they are being held. Even on a success, the victim would avert their eyes. Of course a shout from their friends that "it's an illusion" would quickly reveal it to be nothing more than an image. (Hold Person is a third level spell, requiring a third level illusion for this to work)
Often players will attempt or want to use a figment to mirror the effect of a more powerful spell, e.g. making a stairway appear infinite so the person just keeps stepping on the same step. But that spell is Maze and figment isn't designed to replace other spells. Figment spells make non-solid holograms and that is their strength and weakness.
The original intent of Figment spells was to create illusionary soldiers and troops (the apocryphal 'force' from Phantasmal Force)—so using it for that purpose should be encouraged and successful.
Other Creative Figments
An impromptu wall can hide an entire party.
An image of a bucket over a monster's head might give him some problems (first level light can grant a -4 to hit, which is equivalent to blindness)
A combination of Change/Alter Self and one of the Figment spells can create halos, glowing lights, subtle music, glowing eyes, etc. providing excellent bonuses or triggering morale rolls.
An Illusionary opponent can engage with someone, but when hit, a save must be made for interaction. (Concentration allows the illusionist to make the creature appear to react realistically). A good rule of thumb for the maximum AC of such an opponent is 10 - the illusionists level - the level of the spell (or + for ascending AC systems) Or armor class 0 (20) whichever is worst. Most require at least some auditory components to be believed, requiring an Improved Phantasmal Force (Minor Image) or greater.
Creating an illusion of a dangerous creature (dragon head, tentacle, giant hand) reaching out from a door or pit to scare opponents, into an actual dangerous area.
The Figment spells are specifically used to enhance things that are already happening. You can trigger a morale roll earlier by making the party and number of attacks appear greater than they are (archers or people throwing javelins). You can use it to make it appear that more enemies (illusory ones) are dying, or that the players are more powerful or dangerous than they are making the morale roll less likely to succeed.
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