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