[RPG] How to determine whether a spell attack hits

attackdnd-5espells

Just starting D&D, with my kids, playing the Lost Mine of Phandelver from the Starter Set.

For a spell attack — let's say on a goblin — it usually says "on a hit" then [xyz] happens to the goblin. But how do I know if the spell has hit the goblin?

OK, first I roll the d20, plus appropriate modifiers for my wizard. But then what am I comparing the resulting number to? Is it to the goblin's armour class? Or to something else?

Best Answer

Anything referred to as an "attack roll" is made against the AC of the target.

Whenever you make an attack roll, whether that's with a weapon or a spell, you are rolling against the Armour Class of the thing you're attacking. I don't have access to the cut-down rulebook that comes in the Lost Mine of Phandelver starter set so I cannot reference that or tell you exactly what the wording in there is, but in the Basic Rules we have these rules about making attack rolls (PHB pg 193-194):

Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

...

When you make an attack, your attack roll determines whether the attack hits or misses. To make an attack roll, roll a d20 and add the appropriate modifiers. If the total of the roll plus modifiers equals or exceeds the target's Armor Class (AC), the attack hits. The AC of a character is determined at character creation, whereas the AC of a monster is in its stat block.

This last part is somewhat dubious because there are lots of things that can change a character's Armour Class, like putting on or taking off armour, casting spells that affect AC, or gaining the benefit of certain magic items (options which are also often available to the monsters/NPCs that the characters will fight, too). I think the starter set you're playing probably avoids that as much as possible though to simplify character management for beginners.

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