Spellcasting
Source: Xanathar’s Guide to Everything p. 85
This section expands on the spellcasting rules presented in the “Player’s Handbook” and the “Dungeon Master’s Guide”, providing clarifications and new options.
Perceiving a Caster at Work
Many spells create obvious effects: explosions of fire, walls of ice, teleportation, and the like. Other spells, such as charm person, display no visible, audible, or otherwise perceptible sign of their effects, and could easily go unnoticed by someone unaffected by them. As noted in the “Player’s Handbook”, you normally don’t know that a spell has been cast unless the spell produces a noticeable effect.
But what about the act of casting a spell? Is it possible for someone to perceive that a spell is being cast in their presence? To be perceptible, the casting of a spell must involve a verbal, somatic, or material component. The form of a material component doesn’t matter for the purposes of perception, whether it’s an object specified in the spell’s description, a component pouch, or a spellcasting focus.
If the need for a spell’s components has been removed by a special ability, such as the sorcerer’s Subtle Spell feature or the Innate Spellcasting trait possessed by many creatures, the casting of the spell is imperceptible. If an imperceptible casting produces a perceptible effect, it’s normally impossible to determine who cast the spell in the absence of other evidence.
Identifying a Spell
Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it’s being cast, or they can use an action on their turn to identify a spell by its effect after it is cast.
If the character perceived the casting, the spell’s effect, or both, the character can make an Intelligence (Arcana) check with their reaction or action. The DC equals 15 + the spell’s level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren’t associated with any class when they’re cast, such as when a monster uses its Innate Spellcasting trait.
This Intelligence (Arcana) check represents the fact that identifying a spell requires a quick mind and familiarity with the theory and practice of casting. This is true even for a character whose spellcasting ability is Wisdom or Charisma. Being able to cast spells doesn’t by itself make you adept at deducing exactly what others are doing when they cast their spells.
Invalid Spell Targets
A spell specifies what a caster can target with it: any type of creature, a creature of a certain type (humanoid or beast, for instance), an object, an area, the caster, or something else. But what happens if a spell targets something that isn’t a valid target? For example, someone might cast charm person on a creature believed to be a humanoid, not knowing that the target is in fact a vampire. If this issue comes up, handle it using the following rule.
If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.
Areas of Effect on a Grid
The Dungeon Master’s Guide includes the following short rule for using areas of effect on a grid.
Choose an intersection of squares as the point of origin of an area of effect, then follow the rules for that kind of area as normal (see the ""Areas of Effect"" section in chapter 10 of the “Player’s Handbook”). If an area of effect is circular and covers at least half a square, it affects that square.
That rule works, but it can require a fair amount of on-the-spot adjudication. This section offers two alternatives for determining the exact location of an area: the template method and the token method. Both of these methods assume you’re using a grid and miniatures of some sort. Because these methods can yield different results for the number of squares in a given area, it’s not recommended that they be combined at the table—choose whichever method you and your players find easier or more intuitive.
Template Method
The template method uses two-dimensional shapes that represent different areas of effect. The aim of the method is to accurately portray the length and width of each area on the grid and to leave little doubt about which creatures are affected by it. You’ll need to make these templates or find premade ones.
Making a Template
Making a template is simple. Get a piece of paper or card stock, and cut it in the shape of the area of effect you’re using. Every 5 feet of the area equals 1 inch of the template’s size. For example, the 20-foot-radius sphere of the fireball spell, which has a 40-foot diameter, would translate into a circular template with an 8-inch diameter.
Using a Template
To use an area-of-effect template, apply it to the grid. If the terrain is flat, you can lay it on the surface; otherwise, hold the template above the surface and take note of which squares it covers or partially covers. If any part of a square is under the template, that square is included in the area of effect. If a creature’s miniature is in an affected square, that creature is in the area. Being adjacent to the edge of the template isn’t enough for a square to be included in the area of effect; the square must be entirely or partly covered by the template.
You can also use this method without a grid. If you do so, a creature is included in an area of effect if any part of the miniature’s base is overlapped by the template.
When you place a template, follow all the rules in the “Player’s Handbook” for placing the associated area of effect. If an area of effect, such as a cone or a line, originates from a spellcaster, the template should extend out from the caster and be positioned however the caster likes within the bounds of the rules.
Diagrams 2.1 and 2.2 show the template method in action.
Token Method
The token method is meant to make areas of effect tactile and fun. To use this method, grab some dice or other tokens, which you’re going to use to represent your areas of effect.
Rather than faithfully representing the shapes of the different areas of effect, this method gives you a way to create square-edged versions of them on a grid easily, as described in the following subsections.
Using Tokens
Every 5-foot square of an area of effect becomes a die or other token that you place on the grid. Each token goes inside a square, not at an intersection of lines. If an area’s token is in a square, that square is included in the area of effect. It’s that simple.
Diagrams 2.3 through 2.6 show this method in action, using dice as the tokens.
Circles
This method depicts everything using squares, and a circular area of effect becomes square in it, whether the area is a sphere, cylinder, or radius. For instance, the 10-foot radius of flame strike, which has a diameter of 20 feet, is expressed as a square that is 20 feet on a side, as shown in diagram 2.3. Diagram 2.4 shows that area with total cover inside it.
Cones
A cone is represented by rows of tokens on the grid, extending from the cone’s point of origin. In the rows, the squares are adjoining side by side or corner to corner, as shown in diagram 2.5. To determine the number of rows a cone contains, divide its length by 5. For example, a 30-foot cone contains six rows.
Here’s how to create the rows. Starting with a square adjacent to the cone’s point of origin, place one token. The square can be orthogonally or diagonally adjacent to the point of origin. In every row beyond that one, place as many tokens as you placed in the previous row, plus one more token. Place this row’s tokens so that their squares each share a side with a square in the previous row. If the cone is orthogonally adjacent to the point of origin, you’ll have one more token to place in the row; place it on one end or the other of the row you just created (you don’t have to pick the side chosen in diagram 2.5). Keep placing tokens in this way until you’ve created all of the cone’s rows.
Lines
A line can extend from its source orthogonally or diagonally, as shown in diagram 2.6.