One of the druids in my group have the spell Ice Knife which specifies:
Make a ranged spell attack against the target. On a hit, the target
takes 1d10 piercing damage. Hit or miss, the shard then explodes. The
target and each creature within 5 feet of the point where the ice
exploded must succeed on a Dexterity saving throw or take 2d6 cold
damage.
When making the first roll he rolled a natural 20. The piercing damage of the knife has the critical hit effect.
Does the explosion of the shard also benefit from the increased damage from the critical hit? (4d6 rather than 2d6)
Best Answer
The damage will remain 2d6 if the save is failed.
Where a spell effect calls for a saving throw, critical damage is not in play. Reviewing spells like lightning bolt, fireball, and thunderwave that have multiple dice for damage, nowhere does the spell effect describe a doubling of damage dice for critical hits.
What this indicates is that the explosion damage is not linked to the attack roll's success or failure, and thus cannot benefit from the auto hit or critical on a 20, nor the auto miss on a 1. (p. 194 PHB)
Ice knife is an unusual spell in that it has two separate effects. One effect is an attack roll (hit or miss, and thus eligible for a critical hit benefit), while the other effect is the standard save versus damage.
Compare to other elemental spells
As a point of comparison, see this last part of the spell description (p. 19 Elemental Evil Player's Companion):
This is comparable to Lightning Bolt's similar "at higher levels" language. From PHB p. 255:
That elemental spell effect is also influenced by a saving throw and has no "critical" hit feature.