Dragon's Bane

You empower up to two creatures of your choice within range with the power to destroy even the mightiest of dragons. Whenever a target makes an attack roll before the spell ends, the target can roll a d4 and add the number rolled to the damage dealt against Dragon creatures (Dragons, Dragonborn, Kobolds, etc.). 

3rd level - 2d4

4th level - 3d4

5th level - 4d4

6th level - 5d4