The King's Secret Technique
The quest is unlocked after completing the All Hyrule, United scenario.
"From time to time, King Rhoam heads for a secluded lodge to practice with his claymore. These training retreats might be even more effective if he had something to augment his strength."
"King Rhoam made some powerful elixirs from the ingredients and returned from the lodge with his skills quite improved. No one alive could possibly surpass His Majesty's deftness with claymores."