pf2e.fyi
From Pathfinder Lost Omens Knights of Lastwall, which isn't enabled.

Detonating Spell

FEAT 6
Activity 1 action

Your spell becomes volatile and explosive. If the next action you use is to Cast a Spell that deals damage to a single target and the spell successfully damages that target, the spell explodes, dealing splash damage equal to the rank of the spell cast to adjacent creatures. Unlike normally, this splash damage doesn't apply to the target. The splash damage dealt is of the same type the spell deals.

Pathfinder Lost Omens Knights of LastwallOGL