"Spirit Walk!" Apolaki stated as he used his other sword which is the Kris.
Apolaki's body got enveloped in a mystical mist that sent his body to the spirit plane. A plane of existence present in the mortal realm that is only possible for the dead souls of others to traverse. He managed to do this because the weapon that he has is special.
Apolaki's sword, the Kris, is actually one of the few Spirit Swords available in the game. It is a sword that is in the physical realm but is also in the spiritual realm. It is similar to Adrian's armaments as they are almost the same but still different. Apolaki's sword is actually a creation of a certain mystic when he laid eyes on the wielder of the Soul Sword.