While the Orcs are best known for their melee prowess, there are some that master the magical arts too. Orcish magic tends to be more feral and raw then the kind practiced by humans or gnomes. Orc Warcasters are especially feared for their dreaded battle curse ability, which makes the one who suffers it more vulnerable to weapons made out of bone.
It is mainly because of this magic that Orcish Legions favor bone weapons. Orc casters are often the first target of artillery strikes, if only so that they don’t curse too many of their opponents.