If I cast Shadow Evocation to simulate Fire Shield then creatures who don't disbelieve the illusion will take only 20% damage when they hit me and are affected by the shield. What about the effect of the shield which reduces fire or cold damage by half though? If I intentionally fail the save against the illusion do I get the protection?
The spell says that objects automatically make their save against it, but a Fireball or Scorching Ray is a spell, not an object. Would I only take half damage if the caster of the spell failed a Will save? How about if there's a trap which shoots a Fireball? Would the caster who put the spell into the trap suddenly get a Will save even if he or she wasn't there?
Chill Shield: The flames are cool to the touch. You take only half damage from fire-based attacks. If such an attack allows a Reflex save for half damage, you take no damage on a successful saving throw.
If the disbelieved attack has a special effect other than damage, that effect is one-fifth as strong (if applicable) or only 20% likely to occur.
If a caster shoots a fireball/scorching ray, then I'd have the caster roll a disbelief check against fire shield. If they pass, then I'd have the fire shield character take 10% less damage and have a 20% chance to negate damage on a successful reflex save.
If a trap shoots a fireball, it automatically disbelieves the spell (it is an object).