Spellcasting

To cast a spell, a character requires three things:

Spell Knowledge and Preparation

Arcane Tricksters, Artificers, Bards, Eldritch Knights, Rangers, Sorcerers, Warlocks, and Wizards must know and/or prepare spell formula to cast spells.

These formulae take different forms, depending upon the discipline of the spell-caster, and usually do not translate across classes.

Example: the spell formula for Magic Missile written by Wizards is different from the Sorcerers Magic Missile.

Some classes, like the Artificer, can change known spells by tinkering with their spellcasting focus. Other classes, like Wizards, write down formulae in their spellbooks. The number of known spells can exceed the number of prepared spells.

Arcane Tricksters and Wizards must prepare spells from their list of known spells. For all other classes, there is no distinction between known spells and prepared spells.

Only prepared spells can be cast.

Clerics, Druids, Sorcerers, and Wizards can prepare a number of spells equal to their Spellcasting Attribute bonus + class level.

Example: A 5th-level Wizard with an Intelligence of 16 can prepare 8 spells.

Artificers and Paladins can prepare a number of spells equal to their Spellcasting Attribute bonus + half their class level, rounded down.

Example: A 3rd-level Paladin with a Charisma of 17 can prepare 4 spells.

Arcane Tricksters, Bards, Eldritch Knights, Rangers, and Warlocks have a fixed number of prepared spells depending on their level.

Example: A 5th-level Warlock can prepare 6 spells, regardless of Charisma.

Spell Level

Second, the character must be able to channel magical ability of that level.

Example: a 5th-level Wizard can channel up to 3rd-level spells.

Mana

Lastly, the character must have available Mana.

Using Mana often requires Verbal, Somatic, and/or Material components, which are consumed when Mana is expended. If these words, gestures, or materials are interrupted or interfered with, the spell fizzles and the Mana is lost.

Environmental Effects

Spells which use an Attack Roll normally generate Critical Hits on a roll of 20.

Example: Call Lightning does not trigger Critical Hits, because it has a Saving Throw.

Certain spells have enhanced effects in certain environments, as follows:

A spell with a damage type matching the environment generates a Critical Hit on a 19 or 20 Attack Roll. Likewise, Saving Throws are at Disadvantage.

Example: A roll of 19 with a Javelin of Lightning during a Thunderstorm triggers a Critical Hit on the target for 2d6 Piercing + 8d6 Lightning. The creatures between the thrower and the target do not receive Critical Hits, but they make a Dexterity Save for half damage at Disadvantage.

Wild Magic Sorcerers trigger a Wild Magic Surge on a spellcasting Attack Roll of 20.

Example: A Wild Magic Sorcerer rolls a 20 to Critical Hit a target with a Ray of Enfeeblement during Winternight. He also triggers a Wild Magic Surge.