Yata no Kagami
Obtained from the Sacred Mirror of Amaterasu Special Event or bought from the Medal Shop for 500 Dragon Medals and 20 Black Medals.
An extremely useful sub for higher difficulties as it allows a card to activate its Skill twice on a Straight+.
This is very valuable for any sort of Support card as it not only allows them to activate their Skill twice, but it also locks them out of being used with Enhance cards.
It's worth noting that the activated skills operate at 80% of their usual power.
Yata no Kagami literally means the "The Eight Hand Mirror" and is one of the Three Sacred Treasures of Japan (also known as the Imperial Regalia of Japan).