Heart of the Storm grants you resistance to lightning and thunder damage.
In addition, whenever you start casting a spell of 1st level or higher that deals lightning or thunder damage, stormy magic erupts from you. This eruption causes creature(s) of your choice that you can see (within 10 feet - all creatures, AOE; or 30 feet - single target) to take lightning or thunder damage (choose each time this ability activates) according to the leveled damage table.
Lvl Damage
6 2d6
9 3d6
11 4d6
13 5d6
15 6d6
17 7d6