Here the printer’s manuscript originally read “by the sword those secret combinations”, which didn’t make sense to the 1830 typesetter; he therefore inserted of in pencil after “by the sword”, thus creating a prepositional phrase, “of those secret combinations”. This of is found throughout all the printed editions of the Book of Mormon, yet the of seems quite odd. There is only one example that refers to the sword in a similar way in the singular:
But there the reference is to “their own hands” and not to the name of a group of people, as here in Ether 13:18. (When referring to swords in the plural, we can get such usage, as in Alma 44:18: “their naked skins and their bare heads were exposed to the sharp swords of the Nephites / yea behold they were pierced and smitten yea and did fall exceeding fast before the swords of the Nephites”.)
Another way to interpret the preposition of in Ether 13:18 is as a complement to the past participle slain. In similar cases elsewhere in the text, the verb phrase “slain of X” (where X is a plural noun phrase) means that X were slain, not that X did the slaying:
In the example in Alma 19:21, there is a displaced prepositional phrase headed by of that refers to the antecedent noun phrase; that is, the equivalent reading is “because of the number of their brethren which he had slain at the waters of Sebus”. Here is another example where an of prepositional phrase refers to the antecedent noun phrase:
In this case there are actually two displaced prepositional phrases; the equivalent reading is “and with them a part of those men of Antipus which were not slain”.
So it is theoretically possible that the of prepositional phrase in the current text for Ether 13:18 refers to the antecedent noun phrase; that is, the equivalent reading could be “there were many people of those secret combinations fighting against Coriantumr which were slain by the sword”. However, this interpretation does not really work since verse 18 ends with the resultive clause “that they might obtain the kingdom”—that is, the intent of the secret combinations was to take over the kingdom. This final that- clause would not have been added if the text meant to say that it was those secret combinations that were being destroyed.
The larger passage implies that it is the secret combinations that were slaying many people, or in other words, “the secret combinations” is the agent in the passage. Elsewhere in the text, when referring to people being slain, we have the agentive preposition by when identifying those doing the slaying:
Although none of these agentive examples explicitly mention death by the sword, the original text here in Ether 13:18 probably read “there was many people which was slain by the sword by those secret combinations”. Somehow the second by was accidentally dropped, most likely when Oliver Cowdery copied the text from 𝓞 into 𝓟 since the prepositional phrase “by the sword” ends the line in 𝓟. Probably the preceding by in “by the sword” led to the loss of the second by as Oliver completed the first prepositional phrase headed by by. The 1830 typesetter inserted of for the missing by, but that emendation doesn’t really work.
Don Brugger (personal communication) wonders if the earliest text here might involve the loss of because of, so that it originally read “there was many people which was slain by the sword because of those secret combinations fighting against Coriantumr”. Such a reading will work. There is also support for the syntax of this proposed emendation, where the noun phrase for the because of is a gerundive, as in the original text for Alma 56:10: “because of the enormity of their forces having slain a vast number of our men”. The main problem with accepting because of as the correct reading for the original text here in Ether 13:18 is that the loss of because of seems quite unlikely. There is, to be sure, one case where the conjunction because was omitted (by scribe 2 of 𝓟):
There is also evidence for the momentary loss of of from because of, as in these three examples where Oliver Cowdery made the error:
But there are no examples in the transmission of the text where because of was ever lost. Thus it seems best to accept here in Ether 13:18 the emendation that assumes the loss of a single short preposition, namely by, rather than the longer because of.
Summary: Emend Ether 13:18 by replacing the secondary preposition of with by; the 1830 typesetter’s of isn’t really appropriate, while the preposition by not only works but it also helps explain why it could have been lost (namely, the preceding prepositional phrase was also headed by by).