A critical spell failure happens on the roll of a 1 by the caster, or a 20 by the saving throw target on the first attack roll or save (since spells may have multiple of either).

The blowback is 1d4 damage of the damage type of the spell.
If the spell doesn't have a damage type, the spellcaster suffers the effect of one roll on the table for the Confusion spell until the end of their next turn.

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