When Mormon writes “Zerahemnah retained his sword,” we have another word used in a different sense than expected. “Retain” means to keep, or hold on to. In this verse, it is used with a non-standard meaning of “recover,” or perhaps “retain again.” It is not a meaning used for “retain” in Webster’s 1828 Dictionary.
The importance of the verse is that even though everything has been honorable to this point in the discussion, Zerahemnah does what one might expect a sworn enemy to do. He attacks Moroni. One of Moroni’s soldiers takes a swing at Zerahemnah, removing his scalp. Although this action certainly bears resemblance to the North American Native American practice of scalping (which comes much later in history), it is more likely that it was simply the result and not the intention. That is, it was not a scalping in order to make a point, but it was rather simply the result of the blow.
Rather than be symbolic in itself, the symbolism comes from what the soldier does with it. He declares what is called a simile curse, that is, a curse that likens one thing to another. Thus, “Even as this scalp has fallen to the earth, which is the scalp of your chief, so shall ye fall to the earth except ye will deliver up your weapons of war and depart with a covenant of peace.” The statement is simply repeating Moroni’s part of the oath and it is now adding a visual representation to emphasize the cursing to come should they not accept the oath.
The result was that many did comprehend the futility of their situation and accepted the oath. After entering into a covenant of peace, they were allowed to depart, according to Moroni’s part of the covenant.