Way of the Empty Palm

Acquisition
Signet of Capture:
 * Cantha
 * Ilidus of the Empty Palm (Boreas Seabed (Explorable))
 * Senkai, Lord of the 1,000 Daggers Guild (Boreas Seabed (Explorable), only during The Halcyon Job)

Whether you have the quest or not a boss with the skill will spawn.