I wasn’t sure if the US event Keldeo would transform when transferred to the Japanese Black 2, so naturally I had to try. I traded over Keldeo from my US white game to my JP Black 2 game for a low level riolu (what an unfair trade!). With Keldeo in my party I walked to the right behind Adler’s (アデク) house in Flocessy Town (サンギタウン) and into the Oath Woods. There, in front of the giant scarred-looking boulder, something happened.
「モンスターボールに 入っている Keldeoが でたがっている ようだ⋯⋯! 外に だして あげますか？」
Keldeo wanted out of
his its—creepy genderless unicorn—ball. I obliged and out it popped.
「Keldeoは コバルオン ビリジオン テラキオンの ３匹が 刻みこんだ 決意から しんぴのつるぎを 思い出した！」
From what I can tell, after it checked out the stone it saw Terrakion, Cobalion, and Virizion’s names etched in it and somehow remembered the move secret sword.
After having learned secret sword, Keldeo began to evolve.
「⋯⋯おや！？ Keldeoの 様子が⋯⋯？」
Instead of the usual evolution cutscene I was treated to Keldeo glowing brightly and then the images of Terrakion, Virizion, and Cobalion briefly glowed and faded.
「Keldeoは 戦う 決意も 新たに 覚悟の姿に なった！」
And so it changed into resolution form, popped back in its ball and we went on our merry way.