How do Counterspell and Legendary Resistance interact with a critical hit on a spell attack?

Question

I was wondering about something in D&D 5e:

About Counterspell

If my sorcerer casts a spell and rolls a natural 20 against a magic user (a normal NPC or a boss), can they counterspell it?

Or is this dependent on the DM’s house rules?

Let’s say my sorcerer crits with a 6th-level chain lightning spell against a lich. Would it be possible for the lich to just counterspell and on a success rob my character of the sweeet sweet taste of satisfaction, as the air starts smelling of crispy lich and singed phylactery?

About Legendary Resistance

Legendary resistance is about saving throws, not about whether my sorcerer’s spell attack is higher than the enemy’s AC...

If my sorcerer crits with a spell (that has a spell save) against a boss that has legendary resistance, can that boss still choose to just succeed their save? ... or is this depending on the DM’s rules?

Example

• Let’s say the target is a dragon. It still has at least one legendary resistance left.
• My sorcerer crits while casting disintegrate.
• The spell attack is higher than the dragon’s AC - the spell hits...
• A successful Dex save means no damage.

But as it’s a crit... would the boss get to use the resistance? If rolling a 20 means a "guarantied hit", the boss shouldn’t get the spell save... right?

If my sorcerer has both disintegrate and chain lightning, and the boss hasn’t spent all legendary resistances, then in general I guess it would be smarter to use the chain lightning, because a successful save would still result in SOME damage. But I'm interested in the special case of nat. 20s

(I would state which spell and what level spell slot I will be using, before I roll my dice to see if I hit, so I wouldn't be able to change my choice of spell if I got a 20, but still...)

Answer 1

Counterspell (PHB, pg. 228)

Counterspell has a casting time of 1 Reaction, which you take when you see a creature within 60 feet casting a spell. The description reads:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.

If counterspell is cast, and its success condition is met, the target spellcaster's spell ends; it has no effect. This happens before any attack rolls or saving throws from the spellcaster's spell occur. If you cast a spell, an enemy with counterspell can cast counterspell the moment you begin casting your spell.

Critical Hits and Saving Throws

Critical hits only apply to attack rolls:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. (PHB, pg. 194)

A monster's legendary resistance has nothing to do with attacks or attack rolls at all. Legendary Resistance says:

If the [monster] fails a saving throw, it can choose to succeed instead.

Legendary Resistance only applies to effects which require the creature to make a saving throw.

When you cast a spell which requires a saving throw, you do not roll dice¹, the target rolls for their saving throw, and there is no such thing as a critical on a saving throw. The rules for saving throws can be found on page 194 of the Player's Handbook.

Things to remember:

(1) Counterspell is resolved before any saving throws or spell attack rolls are made.

(2) A spell only requires a spell attack roll if it says it does.

(3) Legendary Resistance only applies to effects that call for a saving throw, not spells that call for a spell attack roll.

I tried to be as concise as possible with this answer and still answer all of your questions. I hope this clears things up.

Let's work out the example you gave.

Let's say your sorcerer is fighting an adult red dragon (MM, pg. 98) with at least one use of its legendary resistance left.

You cast disintegrate on the dragon; disintegrate says:

A creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 10d6 + 40 force damage. (PHB, pg. 233)

Disintegrate calls for the target to make saving throw. No attack roll is made. Your sorcerer cannot crit while casting disintegrate. You cast it, and the dragon makes its saving throw, and if it fails then it can choose to expend a use of legendary resistance to succeed instead. If it succeeds on the save, it takes no damage from disintegrate.

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^{1 An exception to this is the spell Ice Knife}

Answer 2

I believe there are a few misconceptions on how spellcasting works in 5th edition.

The ordering is roughly as follows. Note that they're rough sketches since new steps can always be inserted via more nuanced reaction triggers.

1. Spellcaster begins casting

2. Nearby creatures may cast Counterspell (or take other appropriate reactions to the casting)

3. Spell resolves

Spell Resolution (Attack)

1. Caster rolls the attack roll
2. Mucking occurs (Cutting Words, War God's Blessing, etc.)
3. Hit resolves (Shield, Arcane Deflection, Saving Face, etc.)
4. Damage resolves (Crown/Redemption Paladin abilities, etc.)

Spell Resolution (Saves)

1. Save-roll triggers occur (Slayer's Counter, etc.)
2. Roll saves
3. Mucking occurs (Bardic Inspiration, etc.)
4. Success/Failures resolve
5. Post-roll triggers occur (Indomitable, Saving Face)
6. Spell effects occur

Note that Counterspell prevents the spell from completing, and therefore the resolution of spell effects is not needed.

As far as I know, the only spell with both an attack roll and a save is Plane Shift. In this case, the critical hit only guarantees that the target must make the save (if you don't hit, the target doesn't even need to save).