I have been gone on vacation for a week, and unable to post. In case anyone is still reading I’ll give some more info on the situation.
First however, I want to give an extreme example to show how a PC can believe something is an illusion even though he fails his save. Lets say you have a mischievous gnome illusionist in your party that always summons the same illusionary Ogre to scare off opponents. One day, the gnome gets angry at you and says, “I’m going to summon my Ogre”. He casts the illusion spell. Lets even pretend you have spellcraft so you know what spell its is. The Ogre illusion appears and you fail your save to disbelieved. Must you now fully believe that the Ogre is real and fight/run from it? Must you take illusionary damage when it attacks you?
Give the PC a huge circumstance bonus on his save due to "knowledge".
Now a nat 1 always fails just like a nat 20 always works (for saving throws and attack rolls).
If you wish to ignore the game mechanics that is fine, but like everything in D&D it is supposed to be an abstraction.
Here is some more info on our vampire scenario.
We open the coffin and find the illusionary body, then we decapitate/burn it. Upon realizing the Kama is missing, one party member says, “I don’t think this is the real body”. This gives him a will save, which he fails. He then asks the cleric to detect magic. The DM tries to say the cleric would do no such thing since there is no reason for him to believe illusionary magic is at work.
DM is absoultely correct here.
With the failed saving throw the cleric is now convinced that he was in error about it being an illusion.
Any argument by the player to the contrary is moot. That is the point of the saving throw - to reflect how the PC perceives things.
He had a momentary thought that it was an illusion, but the saving throw (which reflects how he interprets this thought) indicated that he was "mistaken".
After a little arguing, the player gets to cast detect magic.
Mistake on the DM's part.
The "only" reason the PC is casting detect magic is becasue the player still believes it is an illusion while the PC does not.
Absolutely meta-gaming, IMO.
The DM does the typical cop-out move of saying, “Everything is magical. Even the walls and floor” the cleric concentrates on the coffin to pick up a particular school of magic, and gets illusion.
Not necessarily a cop out. It could have been, it all depends on where the cleric is "concentrating" and for how long.
Remember that the spell detect magic works in a specific way depending on where and for how long you are concentrating.
1st rnd presence or absence of magical auras
2nd rnd number of different magical auras and power of the most potent aura (as in faint to overwhelming)
3rd rnd strength and location of each aura. Requires a Spellcraft check to determine the school.
Our rogue does a search on the coffin, rolls pretty well too, and finds nothing. We all determine that there must be something there.
More metagaming. You do not know how successful the PC is on his check.
If that last body wasn’t the real one then that gaseous form had to go somewhere. That’s when I decide to pour water into the coffin to see if it runs out any cracks or holes. This is where the DM says that our investigations must stop. Our failed will saves and search checks mean that we believe nothing is there and cant investigate further.
All predicated on the fact that the players were inserting their personal knowledge into the game mechanics.
One person in our group said he believed that the DMs interpretation of illusions made them to powerful. A low level illusionist could just summon a Pit Fiend and if you failed your will save you were dead. (I don’t think illusions you believe deal illusionary damage, but at least our DM is uses it that way) So he makes the claim that illusionist are now the most powerful class ever, and we should all play one. The DM mentioned that illusions are supposed to be a DM thing anyway.
Wrong on the being dead part. Well for low level illusions anyway - high level ones will cause you to die.
Also it seems like an assumption on the players parts as to how the DM would handle this.
Here is an exedrcise to try. Look at it from the opposite side. The PCs are casting illusions how would the players want the NPCs to handle things if they fail their saving throws?