Elemental Specialist (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast at least one spell
You specialize in casting spells of a specific element. You are able to emulate other damaging spells using your preferred element, but you are unable to deal any other type of spell damage.
When you gain this feat, choose one of the following damage types: acid, cold, fire, lightning, or thunder. Damaging spells you cast deal damage of the chosen type instead of their normal damage.
When you hit a creature that is immune to damage of the chosen type, it takes damage as if it had resistance rather than immunity.
Any creature that absorbs that particular damage type instead takes half of the resisted damage type and does not regain any hit points from it.