Thunderbrids Wrath

You channel the power of Thunderbrid, releasing a powerful blast of storm energy all around. All creatures in a 600 foot radius sphere centered on you must make a DC18 Dexterity saving throw ; taking 40d20 lightning damage, and 20d20 thunder damage on a failure, or half as much on a success. However, the power of Thunderbrid is too great for you to contain, and upon casting this spell, you yourself take 10d20 lightning damage, aswell as 5d20 thunder damage.

For information on the Thunderbrid, visit dndbeyond's homebrew monsters and type Thunderbrid into the search bar. Alternatively, you can follow the link in the comments.