Obtained from the Bastok Markets [S] Quest "Beneath the Mask". Need to be on Mission 26 of Wings of the Goddess "Fate in Haze" and completed the Bastok Markets [S] quest "Honor Under Fire".
I love how SE keeps making more of these awesome potions and sticking them in drop pools from NMs without giving them to Alchemists at all. Awesome Square, you're the best!