What is the purpose of a spell attack modifier?
In the 5e Player's Handbook, the Wizard Class on page 114 defines a spell attack modifier, but the spells I have looked at all specify attack damage without using this modifier or mentioning it. I can't determine what it has to do with anything. Where does it come into the game?
There are two ways that most damage spells work in 5e. You've obviously already found the spell attack modifier calculation. But you're probably looking at a spell wondering how to apply it.
The first is the following form from Fireball:
....Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity Saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.... (PHB 241-242)
This is a spell with a saving throw. The target(s) need to make a Spell Save DC specified by the casting class that the spell belongs to (so if you're a wizard, you use INT, if you're a Sorcerer you use CHA).
For the second, we'll use the example from Fire Bolt:
Make a ranged spell attack against the target. On a hit, the target takes 1d10 fire damage.... (PHB 242)
This is the other common damage spell type. In this case you (as the attacker) roll your d20 and add your spell attack modifier.
Occasionally you'll find a spell that does damage without an attack or save, these spells just simply do their damage.
So if you're trying to figure out whether you damage someone with a damage spell, look for the following:
- Target makes a saving throw (in this case, they use their ability and add it to their d20 roll, and the target number is your spell save DC)
- You make a spell attack (in this case you are rolling your d20, adding your spell attack modifier and comparing it to their AC)
- Damage is simply applied without an attack or save (See Magic Missile).