When Evan received the information about the Halberd of Destruction after bringing it out of the Tomb, he also received the knowledge about using the Halberd properly.
During the two months he spent after coming out of the Tomb, other than increasing his understanding of the conceptual energy of void, he also spent a considerable amount of time practising Halberd techniques.
Besides the basic Halberd techniques, he also received information about a special technique he could execute only while holding the Halberd of Destruction. The special technique was called—Soul Slayer War Arts.
Because the Halberd of Destruction Evan brought out from the Tomb was only a replica, not the real Halberd of Destruction, the Soul Slayer War Arts he received were incomplete.
Evan didn't know how many moves were in the Soul Slayer War Arts, but because he did not have the real Halberd of Destruction, he received information about only the first three moves.