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 within that area. At the beginning of your next turn, the circle detonates, dealing ] 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).