You fire a bolt of magical energy from your hand. Make a ranged spell attack. On a hit, the target takes 6d12 lightning damage and must make a constitution saving throw or be stunned until the end of its next turn. If the target fails its saving throw, it must make the save again at the end of its next turn, or take 6d12 lightning damage and be stunned until the end of its next turn. The effect continues until the target successfully saves against the spell.