Feat: Secondary Detonation Array

You divert some of your spell's energy into an unstable runic array. If your next action is to Cast a Spell that deals damage, has no duration, and affects an area, a glowing magic circle appears in a 5-foot burst within that area. At the beginning of your next turn, the circle detonates, dealing 1d6 force damage per rank of the spell to all creatures within the circle, with a basic Reflex save against your spell DC. If the spell dealt a different type of damage, the circle deals this type of damage instead (or one type of your choice if the spell could deal multiple types of damage).