The spell captures some of the incoming energy, lessening its effect on you and storing it for your next melee attack. You have resistance to the triggering damage type until the start of your next turn. Also, the first time you hit with a melee attack on your next turn, the target takes an extra 1d6 damage of the triggering type, and the spell ends.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the extra damage increases by 1d6 for each slot level above 1st.

Spell Mastery. A DM may reward a spellcaster with a spell mastery - a spell with which they have such prowess that the spell gains additional or alternate effects. Perhaps through sheer frequency of use, the player has developed an innate understanding of how the spell can be manipulated. Perhaps they spent extensive downtime in research or under the tutelage of a powerful mage. Perhaps a spell scroll with lost knowledge of the spell's potential was found in a dragon's hoard. However it is acquired, a spell mastery defies bounded accuracy and should not be granted liberally. A DM may grant one or more spell mastery options for a given spell, but should grant no more than one spell with spell mastery per tier of play.

Spell Mastery 1: You can target another creature within 30 feet, granting them resistance to the triggering damage instead. 
Spell Mastery 2: The damage die for the additional damage taken by the target of your next melee attack increases to a d10.
Spell Mastery 3: The target of this spell instead gains immunity to the triggering damage type until the start of their enxt turn.