Magic is the most complex combat system in D&D, because it interacts with nearly every part of the game. Not all characters can use magic — only certain classes, subclasses, or occasionally species gain access to spellcasting. Each spellcaster uses a spellcasting ability (Intelligence, Wisdom, or Charisma, depending on class) to determine how effective their spells are. This ability affects two key values:
Spell attack bonus = spellcasting ability modifier + proficiency bonus
Spell save DC = 8 + spellcasting ability modifier + proficiency bonus.
If you cast Fire Bolt, you roll a spell attack against the target’s AC. If you cast Hold Person, the target must succeed on a saving throw against your spell save DC or suffer the effect.
Spells are divided into levels, from cantrips (level 0, usable infinitely) to 9th-level spells, which are incredibly powerful and rare. Casting a spell usually consumes a spell slot of that level or higher. Spells also have a casting time (such as an action, bonus action, reaction, or longer), which determines when and how they can be used in combat. For example, Shield is a reaction, while Fireball uses an action.
Every spell also has components:
Verbal (V): speaking magical words (can be blocked by silence
Somatic (S): hand gestures (hands must be free)
Material (M): physical components or a spellcasting focus
Some spells require concentration, meaning you can only maintain one such spell at a time. If you take damage while concentrating, you must succeed on a Constitution saving throw or lose the spell. For example, concentrating on Fly while taking heavy damage is risky.
Spells belong to schools of magic (such as Evocation, Illusion, Necromancy), which describe their theme and purpose. They also deal different damage types (fire, cold, necrotic, radiant, force, etc.), which matter because some creatures resist, are immune to, or are vulnerable to certain types. For example, many undead resist necrotic damage but are vulnerable to radiant damage.
Finally, many spells inflict conditions, which are ongoing effects that significantly alter combat. Examples include:
Blinded: disadvantage on attacks, attacks against you have advantage
Charmed: cannot attack the charmer and may obey them
Frightened: disadvantage on checks and attacks while the source is visible
Paralyzed: incapacitated, automatically fail Strength/Dex saves, attacks against you have advantage and auto-crit at close range
Restrained: speed reduced to 0, disadvantage on attacks and Dexterity saves.
Conditions are extremely powerful and are a major reason spellcasters can control entire fights without dealing direct damage.
