How do you get your fathers spear in Valhalla Knights Eldar saga

The spear, or polearm as called in game, is called a Partizan, and is a Rank 10 weapon, making it the best in its category. You can only find it as random loot. A good method is to purchase very expensive gamble boxes, save, open, and reload, repeating process until you get what you seek. It is a very very very long process. I would know, Ive done it to acquire all the rank 10 weapons and full black set. I can only wish you luck, though weapons are much more common than black set pieces.