Obtained from the Sacred Mirror of Amaterasu Special Event or bought from the Medal Shop for 200 Dragon Medals and 5 Black Medals.
As noted, the Kusanagi Sword is only effective for Attack Skills, but it's a great sub for any attacker.
While it's not quite a 35% Skill Boost, it's fairly close and offers a substantial boost regardless.
Not to mention you still get the 15% Skill Boost even if you don't make a Straight+ hand.
The Kusanagi-no-Tsurugi is a legendary Japanese sword and one of three Imperial Regalia of Japan.