Wrath of the Storm is a 
passive feature. It grants the Cleric two reactions to strike back at an attacking creature, doing Lightning damage or Thunder damage.
Description
As a reaction, strike back at an attacking creature, dealing 2d8
Lightning or 2d8
Thunder damage. The target takes half damage on a successful 
Saving throw.
How to learn
Classes:
- Class level 1: Tempest Domain
Notes
For extra clarity, attacks here mean a successful hit. Any damage dealt from a missed attack will not trigger the effect.