Needs review
Source: Complete Arcane

Description: You can modify a spell that uses one type of energy to add an equal amount of another energy type.

Prerequisites: Energy Substitution

Choose one type of energy (acid, cold, electricity, or fire) that matches an energy type you have selected for substitution via the Energy Substitution feat. You can then modify any spell with an energy descriptor by adding an equal amount of the chosen type of energy to the spell's normal effects. The altered spell works normally in all respects except for the type and amount of damage dealt, with each type of energy counting separately toward the spell's damage cap. Thus, an acid fireball cast at 6th level deals 6d6 points of fire damage and 6d6 points of acid damage (rolled separately), while the same acid fireball cast at 10th level or higher deals 10d6 points of fire damage and 10d6 points of acid damage. Even opposed types of energy (such as fire and cold) can be combined using this feat. An energy admixed spell uses up a spell slot four levels higher than the spell's actual level. As well, the spell's descriptor changes to include both energy types present in the spell—for example, the acid fireball described above is an evocation [acid, fire] spell.