Tempest Domain clerics have the ability Wrath of the Storm (PHB, p. 62):
When a creature within 5 feet of you that you can see hits you with an attack, you can use your reaction to cause the creature to make a Dexterity saving throw. The creature takes 2d8 lightning or thunder damage (your choice) on a failed saving throw, and half as much damage on a successful one.
It doesn't specify what the DC of the saving throw is.
I assume it uses the spell save DC, even though it technically isn't a spell?
Spell save DC = 8 + your proficiency bonus + your Wisdom modifier
Best Answer
Cleric's Spell Save DC
You are correct that RAW the saving DC is not specified. I have been unable to find any source that officially corrects this oversight. However, I think using the cleric's spell save DC is a fairly safe assumption.
Most cleric features that require a save come under the Channel Divinity set of features. In the text for that feature we have:
Technically Wrath of the Storm is neither a spell nor a Channel Divinity effect so this isn't RAW. I believe we can draw an effective parallel to use this a precedent though. Additionally, wisdom is listed as the primary ability for clerics so it is a reasonable assumption to default to this when a class feature is lacking an exact definition.