Your spirit has been charged by a draconic ancestry in your past. On rare occasions, you are able to identify the brief seconds before a dragon unleashes its breath weapon at you and can react, your muscles and arcane ability able to respond with uncanny speed. As part of this charge of spirit, you learn the spell "Lightning Bolt".
When a dragon you can see uses it's breath weapon against you, if the breath weapon has a saving throw, once per long rest you can use your reaction to switch the saving throw to your primary spellcasting ability save. If you save versus the dragon's breath weapon DC, you may immediately cast a 4th level "Lightning Bolt" at the same time as the incoming breath weapon.
The dragon may make a DEX saving throw versus the your spell save DC, per the Lighting Bolt spell. Roll the Lightning Bolt's damage and the dragon will roll its breath weapon damage. Whatever damage the dragon would have taken from the Lightning Bolt is instead thrown against the breath weapon in a collision of arcane might. If any damage is left over the damage is dealt. For example, if your Lightning Bolt deals 80 damage, but the dragon saves for half, the spell negates 40 points of breath weapon damage. If the breath weapon only dealt 30 points of damage, the dragon take 10 lightning damage as your Lighting Bolt shatters the breath weapon and slams into the dragon. If the breath weapon dealt 50 damage, you take 10 breath weapon damage.
Creatures in the path of the breath weapon who are within 30 ft of you, also benefit from the damage negation.
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