Pathfinder - How to Play - Casting a Spell
Aiming a Spell
You must make choices about whom a spell is to affect or where an effect is to originate, depending on a spell's type. The next entry in a spell description defines the spell's target (or targets), its effect, or its area, as appropriate.
Remember that spells affect your allies unless the spell description allows you to specify specific targets or specifies that it only affects enemies. Be careful while aiming spells so that you don't harm your allies.
Target or Targets
Some spells have a target or targets. You cast these spells on creatures or objects, as defined by the spell itself. You must be able to see or touch the target, and you must specifically choose that target. You do not have to select your target until you finish casting the spell.
If the target of a spell is yourself (the Target line of the spell description includes "You"), you do not receive a saving throw, and spell resistance does not apply. The saving throw and spell resistance lines are omitted from such spells.
Some spells restrict you to willing targets only. Declaring yourself as a willing target is something that can be done at any time (even if you're flat-footed or it isn't your turn). Unconscious creatures are automatically considered willing, but a character who is conscious but immobile or helpless (such as one who is bound, cowering, grappling, paralyzed, pinned, or stunned) is not automatically willing.
Some spells allow you to redirect the effect to new targets or areas after you cast the spell. Redirecting a spell is a move action that does not provoke attacks of opportunity.
Some spells create or summon things rather than affecting things that are already present. You must designate the location where these things are to appear, either by seeing it or defining it. Range determines how far away an effect can appear, but if the effect is mobile, after it appears it can move regardless of the spell's range, unless the spell specifies otherwise.
Some effects are rays. You aim a ray as if using a ranged weapon, though typically you make a ranged touch attack rather than a normal ranged attack. As with a ranged weapon, you can fire into the dark or at an invisible creature and hope you hit something. You don't have to see the creature you're trying to hit, as you do with a targeted spell. Intervening creatures and obstacles, however, can block your line of sight or provide cover for the creature at which you're aiming.
If a ray spell has a duration, it's the duration of the effect that the ray causes, not the length of time the ray itself persists.
If a ray spell deals damage, you can score a critical hit just as if it were a weapon. A ray spell threatens a critical hit on a natural roll of 20 and deals double damage on a successful critical hit. This includes spells that deal ability damage, so if you score a critical hit which does ability damage (such as Chill Touch, whihc does both hit point damage and Strength damage), the ability damage is also doubled.
Some effects, notably clouds and fogs, spread out from a point of origin, which must be a grid intersection. The effect can extend around corners and into areas that you can't see. Figure distance by actual distance traveled, taking into account turns the spell effect takes. When determining distance for spread effects, count around walls, not through them. As with movement, do not trace diagonals across corners. You must designate the point of origin for such an effect, but you need not have line of effect (see below) to all portions of the effect.
Some spells affect an area. Sometimes a spell description specifies a specially defined area, but usually an area falls into one of the categories defined below.
Regardless of the shape of the area, you select the point where the spell originates, but otherwise you don't control which creatures or objects the spell affects. The point of origin of a spell is always a grid intersection. When determining whether a given creature is within the area of a spell, count out the distance from the point of origin in squares just as you do when moving a character or when determining the range for a ranged attack. The only difference is that instead of counting from the center of one square to the center of the next, you count from intersection to intersection.
You can count diagonally across a square, but remember that every second diagonal counts as 2 squares of distance. If the far edge of a square is within the spell's area, anything within that square is within the spell's area. If the spell's area only touches the near edge of a square, however, anything within that square is unaffected by the spell.
Burst, Emanation, or Spread
Most spells that affect an area function as a burst, an emanation, or a spread. In each case, you select the spell's point of origin and measure its effect from that point.
A burst spell affects whatever it catches in its area, including creatures that you can't see. It can't affect creatures with total cover from its point of origin (in other words, its effects don't extend around corners). The default shape for a burst effect is a sphere, but some burst spells are specifically described as cone-shaped. A burst's area defines how far from the point of origin the spell's effect extends.
An emanation spell functions like a burst spell, except that the effect continues to radiate from the point of origin for the duration of the spell. Most emanations are cones or spheres.
A spread spell extends out like a burst but can turn corners. You select the point of origin (which is an intersection, not a square), and the spell spreads out a given distance in all directions. Figure the area the spell effect fills by taking into account any turns the spell effect takes.
Cone, Cylinder, Line, or Sphere
Most spells that affect an area have a particular shape.
A cone-shaped spell shoots away from you in a quarter-circle in the direction you designate. It starts from any corner of your square and widens out as it goes. Most cones are either bursts or emanations (see above), and thus won't go around corners.
Note that the example 15 foot cone layouts pointing straight up from the caster are somewhat in conflict with the rules. According to this post on the Paizo rules questions forum, these discrepancies are by design. Yes, this is confusing. I am very sorry for this, and I wish that Paizo would find a better way to measure cones. For every other cone width, use my diagram because
When casting a cylinder-shaped spell, you select the spell's point of origin. This point is the center of a horizontal circle, and the spell shoots down from the circle, filling a cylinder. A cylinder-shaped spell ignores any obstructions within its area. Use the diagram for Burst, Emanation, and Spread effects to determine the horizontal shape of the cylinder.
A line-shaped spell shoots away from you in a line in the direction you designate. It starts from any corner of your square and extends to the limit of its range or until it strikes a barrier that blocks line of effect. A line-shaped spell affects all creatures in squares through which the line passes.
A sphere-shaped spell expands from its point of origin to fill a spherical area. Spheres may be bursts, emanations, or spreads. Use the diagram for Burst, Emanation, and Spread effects to determine the horizontal shape of the cylinder.
A spell with this kind of area affects creatures directly (like a targeted spell), but it affects all creatures in an area of some kind rather than individual creatures you select. The area might be a spherical burst, a cone-shaped burst, or some other shape.
Many spells affect "living creatures," which means all creatures other than constructs and undead. Creatures in the spell's area that are not of the appropriate type do not count against the creatures affected.
A spell with this kind of area affects objects within an area you select (as Creatures, but affecting objects instead).
A spell can have a unique area, as defined in its description.
If an area or effect entry ends with "(S)," you can shape the spell. A shaped effect or area can have no dimension smaller than 10 feet. Many effects or areas are given as cubes to make it easy to model irregular shapes. Three-dimensional volumes are most often needed to define aerial or underwater effects and areas.
Line of Effect
A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight. Solid, transparent barriers like glass block line of effect.
You must have a clear line of effect to any target that you cast a spell on or to any space in which you wish to create an effect. You must have a clear line of effect to the point of origin of any spell you cast.
A burst, cone, cylinder, or emanation spell affects only an area, creature, or object to which it has line of effect from its origin (a spherical burst's center point, a cone-shaped burst's starting point, a cylinder's circle, or an emanation's point of origin). Full cover, such as a solid wall, blocks the effect.
An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect. Such an opening means that the 5-foot length of wall containing the hole is no longer considered a barrier for purposes of a spell's line of effect.
Spells and Precision Damage
Any spell which requires an attack roll and deals damage (hit point damage or ability damage) can apply precision damage from abilities like Sneak Attack. As normal, precision damage and additional damage dice are not multiplied on a critical hit. If the spell allows multiple attack rolls (such as Scorching Ray, which allows one ray per four caster levels), you may apply precision damage such as Sneak Attack on all of the attacks, provided you meet the normal conditions for doing so.