Using this variant set of rules, a spellcaster does not automatically succeed when casting a spell. Instead, the caster must make a caster level check (D20+ Caster Level+ Characteristic Modifier). The DC for this check is 10+ the spell's level. If this check is passed, the spell is cast successfully as normal. If it fails, the spell is not cast (it takes no more than a standard action to fail a casting attempt). If a natural one is rolled, then the spell goes horribly wrong. See the section on Critical Spell Failures for details of the result. If a natural twenty is rolled, then the spell is a critical success. See the section on critical successes for details.