I know there should have been no save needed the second we tried to decapitate and burn the corpse, as our swords would just pass through it. Our DM says that if you interact with an illusion the way the illusion expects you to, you get no save and its all believable. I have a hard time arguing with him since the module put this illusion there to fool us into thinking we had killed the vampire. It would not be a very good trick if the illusion went away the second we touched it.
And after 3 pages of posts, I am still not getting the “out of character” and “metagaming” arguments. Our DM says you only get a save when you have reason to disbelieve; interaction gives you nothing. So by the time we started saying that something was wrong, we had already decapitated and burned this body. We felt the head cut off, we saw the body burn to dust. Every bit of physical evidence that the players and their PCs had said that the body was real. Yet, despite all that, we didn’t completely believe the corpse was real based on our own intuition. No one even really brought up illusion until the DM said “make a disbelieve check” We were all just thinking it was a dummy corpse.
It just seems a little overpowered the way the DM is running it. We just completed that dungeon and we left that vampire room never to return. Because now that we have failed our Will saves, we will never go back and never truly destroy that vampire.
If you are feeling the head after it was decapitated, then it was not a permanent image or programmed image. A permanent or programmed image is not designed to fool anybody up close for a long time.
Interacting is any sort of interaction - be it combat, talking, casting a spell on it, etc. That sort of spell in a coffin should be designed to fool a careless onlooker ("ok, it needs to stay in its coffin for 2 more hours, lets move on to the next room and come back later to finish it off...") or to slow down pursuers for a round or three so it can make good its escape to its real coffin.
And, unless the vampire is there hidden and pulling the illusion's strings, how can it know if you will stake it through the heart, chop off its head, douse it with Holy Water, or what. What if you decided to knock over the coffin & dump the body onto the floor instead? A
permanent image is a static effect, though it can be moved if the vampire is concentrating on it. A
programmed image only functions after a trigger (opening the coffin?) and will do one function per the spell. Would the vampire have the programmed image react to its neck being touched? What happens if you also pour holy water on the image first? Or, toss cloves of garlic, or smack it with your Holy Symbol? Heck, I've known a PC or two in my 30 years of gaming that would then stand over the beheaded corpse and urinate on it.
It seems to me the DM thinks illusions are far more powerful than they really are. Even level 7
Project Image does not create a tangible image, though a projected image can interact with the PCs if controlled by the vampire. And, a projected image needs line of sight to function. If you swing your weapon at an illusion like a projected, permanent or programmed image, it will pass through the image. If your DM does not rule that an automatic failure, then the PC should at least be able to realize something unusual is up with the corpse. And, even before the weapon swing, the swinging PC is close enough to get a Will save to disbelieve.