The ability is obtained as a reward for completing the quest Dragon Research given by Esbern. Upon completion of the quest Esbern gives the Dragonborn Esbern's Potion that must be drunk in order to permanently receive the ability.
"Dragons do 25% less melee damage."
The ability is currently bugged as upon drinking the potion the ability is not granted in the active effects menu.
- This bug has been fixed by the official patch 1.9 for new and existing characters.
player.addspell f5ffa Solution: add the perk manually using the console command