Blue Dragon ancestry: Blue Dragonborn trace their ancestry to a Blue Dragon, granting them a magical affinity for lightning.

Breath weapon: When Blue Dragonborn take the Attack action on their turn, they can replace one of their attacks with an exhalation of magical energy in a 30-foot line that is 5 feet wide. Each creature in that area must make a Dexterity saving throw (DC = 8 + Constitution modifier + proficiency bonus). On a failed save, the creature takes 1d10 lightning damage. On a successful save, it takes half as much damage. This damage increases by 1d10 when they reach 5th level (2d10), 11th level (3d10), and 17th level (4d10). They can use their Breath Weapon a number of times equal to their proficiency bonus, and they regain all expended uses when they finish a long rest.

Draconic resistance: Blue Dragonborn have resistance to lightning damage.

Lightning warding: Starting at 5th level, as an action, Blue Dragonborn can channel their draconic energy to protect themselves. For 1 minute, they become immune to lightning damage. Once they use this trait, they can’t do so again until they finish a long rest.