Illusions
Of all spells, those of the Illusion school cause the most problems. Not that they are more difficult for your player character to cast, but these spells are more difficult for you to role-play and for your DM to adjudicate. Illusions rely on the idea of believability, which in turn relies on the situation and the state of mind of the victim. Your DM must determine this for NPCs, which is perhaps an easier job. You must role-play this for your character.
Spells of this school fall into two basic groups. Illusions are creations that manipulate light, color, shadow, sound, and sometimes even scent. Higher level illusions tap energy from other planes, and are actually quasi-real, being woven of extradimensional energies by the caster. Common illusions create appearances; they cannot make a creature or object look like nothing (i.e., invisible), but they can conceal objects by making them look like something else.
Phantasms exist only in the minds of their victims; these spells are never even quasi-real. (The exceptions to this are the phantasmal force spells, which are actually illusions rather than phantasms.) Phantasms act upon the mind of the victim to create an intense reaction--fear being most common.
The key to successful illusions or phantasms is believability, which depends on three main factors: what the caster attempts, what the victim expects, and what is happening at the moment the spell is cast. By combining the information from these three areas, the player and the DM should be able to create and adjudicate reasonable illusions and phantasms.
When casting an illusion or phantasm, the caster can attempt to do anything he desires within the physical limits of the spell. Prior knowledge of the illusion created is not necessary but is extremely useful.
Suppose Delsenora decides to cast a phantasmal force spell and can choose between creating the image of a troll (a creature she has seen and battled) or that of a beholder (a creature she has never seen but has heard terrifying descriptions of). She can either use her memory to create a realistic troll or use her imagination to create something that may or may not look like a real beholder. The troll, based on her first-hand knowledge of these creatures, is going to have lots of little details--a big nose, warts, green, scabby skin, and even a shambling troll-like walk. Her illusion of a beholder will be much less precise, just a floating ball with one big eye and eyestalks. She doesn't know its color, size, or behavior.
The type of image chosen by the caster affects the reaction of the victim. If the victim in the above case has seen both a troll and a beholder, which will be more believable? Almost certainly it will be the troll, which looks and acts the way the victim thinks a troll should. He might not even recognize the other creature as a beholder since it doesn't look like any beholder he's ever seen. Even if the victim has never seen a troll or a beholder, the troll will still be more believable; it acts in a realistic manner, while the beholder does not. Thus, spellcasters are well-advised to create images of things they have seen, for the same reason authors are advised to write about things they know.
The next important consideration is to ask if the spell creates something that the victim expects. Which of these two illusions would be more believable--a huge dragon rising up behind a rank of attacking kobolds (puny little creatures) or a few ogres forming a line behind the kobolds? Most adventurers would find it hard to believe that a dragon would be working with kobolds. The dragon is far too powerful to associate with such little shrimps. Ogres, however, could very well work with kobolds--bossing them around and using them as cannon fodder. The key to a good illusion is to create something the victim does not expect but can quickly accept.
The most believable illusion may be that of a solid wall in a dungeon, transforming a passage into a dead end. Unless the victim is familiar with these hallways, he has no reason not to believe that the wall is there.
Of course, in a fantasy world many more things can be believed than in the real world. Flames do not spring out of nowhere in the real world, but this can happen in a fantasy world. The presence of magic in a fantasy world makes victims more willing to accept things our logic tells us cannot happen. A creature appearing out of nowhere could be an illusion or it could be summoned. At the same time, you must remember that a properly role-played character is familiar with the laws of his world. If a wall of flames appears out of nowhere, he will look for the spellcaster. A wall blocking a corridor may cause him to check for secret doors. If the illusion doesn't conform to his idea of how things work, the character should become suspicious. This is something you have to provide for your character and something you must remember when your character attempts to use illusions.
This then leads to the third factor in the believability of an illusion, how appropriate the illusion is for the situation. As mentioned before, the victim is going to have certain expectations about any given encounter. The best illusions reinforce these expectations to your character's advantage. Imagine that your group runs into a war party of orcs in the local forest. What could you do that would reinforce what the orcs might already believe? They see your group, armed and ready for battle. They do not know if you are alone or are the advance guard for a bigger troop. A good illusion could be the glint of metal and spear points coming up behind your party. Subtlety has its uses. The orcs will likely interpret your illusion as reinforcements to your group, enough to discourage them from attacking.
However, the limitations of each spell must be considered when judging appropriateness. A phantasmal force spell creates vision only. It does not provide sound, light, or heat. In the preceding situation, creating a troop of soldiers galloping up behind you would not have been believable. Where is the thunder of hooves, the creak of saddle leather, the shouts of your allies, the clank of drawn metal, or the whinny of horses? Orcs may not be tremendously bright, but they are not fooled that easily. Likewise, a dragon that suddenly appears without a thunderous roar and dragonish stench isn't likely to be accepted as real. A wise spellcaster always considers the limitations of his illusions and finds ways to hide their weaknesses from the enemy.
An illusion spell, therefore, depends on its believability. Believability is determined by the situation and a saving throw. Under normal circumstances, those observing the illusion are allowed a saving throw vs. spell if they actively disbelieve the illusion. For player characters, disbelieving is an action in itself and takes a round. For NPCs and monsters, a normal saving throw is made if the DM deems it appropriate. The DM can give bonuses or penalties to this saving throw as he thinks appropriate. If the caster has cleverly prepared a realistic illusion, this certainly results in penalties on the victim's saving throw. If the victim were to rely more on scent than sight, on the other hand, it could gain bonuses to its saving throw. If the saving throw is passed, the victim sees the illusion for what it is. If the saving throw is failed, the victim believes the illusion. A good indication of when player characters should receive a positive modifier to their saving throws is when they say they don't believe what they see, especially if they can give reasons why.
There are rare instances when the saving throw may automatically succeed or fail. There are times when the illusion created is either so perfect or so utterly fantastic as to be impossible even in a fantasy world. Be warned, these occasions are very rare and you should not expect your characters to benefit from them more than once or twice.
In many encounters, some party members will believe an illusion while others see it for what it really is. In these cases, revealing the truth to those deluded by the spell is not a simple matter of telling them. The magic of the spell has seized their minds. Considered from their point of view, they see a horrible monster (or whatever) while a friend is telling them it isn't real. They know magic can affect people's minds, but whose mind has been affected in this case? At best, having an illusion pointed out grants another saving throw with a +4 bonus.
Illusions do have other limitations. The caster must maintain a show of reality at all times when conducting an illusion. (If a squad of low-level fighters is created, the caster dictates their hits, misses, damage inflicted, apparent wounds, and so forth, and the referee decides whether the bounds of believability have been exceeded.) Maintaining an illusion normally requires concentration on the part of the caster, preventing him from doing other things. Disturb him and the illusion vanishes.
Illusions are spells of trickery and deceit, not damage and destruction. Thus, illusions cannot be used to cause real damage. When a creature is caught in the blast of an illusionary fireball or struck by the claws of an illusionary troll, he thinks he takes damage. The DM should record the illusionary damage (but tell the player his character has taken real damage). If the character takes enough damage to "die," he collapses in a faint. A system shock roll should be made for the character. (His mind, believing the damage to be real, may cause his body to cease functioning!) If the character survives, he regains consciousness after 1d3 turns with his illusionary damage healed. In most cases, the character quickly realizes that it was all an illusion.
When an illusion creates a situation of inescapable death, such as a giant block dropping from the ceiling, all those believing the illusion must roll for system shock. If they fail, they die--killed by the sheer terror of the situation. If they pass, they are allowed a new saving throw with a +4 bonus. Those who pass recognize the illusion for what it is. Those who fail faint for 1d3 turns.
Illusions do not enable characters to defy normal physical laws. An illusionary bridge cannot support a character who steps on it, even if he believes the bridge is real. An illusionary wall does not actually cause a rock thrown at it to bounce off. However, affected creatures attempt to simulate the reality of what they see as much as possible. A character who falls into an illusionary pit drops to the ground as if he had fallen. A character may lean against an illusionary wall, not realizing that he isn't actually putting his weight on it. If the same character were suddenly pushed, he would find himself falling through the very wall he thought was solid!
Illusions of creatures do not automatically behave like those creatures, nor do they have those creatures' powers. This depends on the caster's ability and the victim's knowledge of the creatures. Illusionary creatures fight using the caster's combat ability. They take damage and die when their caster dictates it. An illusory orc could continue to fight, showing no damage, even after it had been struck a hundred or a thousand times. Of course, long before this its attackers will become suspicious. Illusionary creatures can have whatever special abilities the caster can make appear (i.e., a dragon's fiery breath or a troll's regeneration), but they do not necessarily have unseen special abilities. There is no way a caster can create the illusion of a basilisk's gaze that turns people to stone. However, these abilities might be manifested through the fears of the victims. For example, Rath the fighter meets an illusionary basilisk. Rath has fought these beasties before and knows what they can do. His gaze accidentally locks with that of the basilisk. Primed by his own fears, Rath must make a system shock roll to remain alive. But if Rath had never seen a basilisk and had no idea that the creature's gaze could turn him to stone, there is no way his mind could generate the fear necessary to kill him. Sometimes ignorance is bliss!
