When I look up 魔力 in the dictionary it gives me the definition of "magical powers". However, when 魔力 is used in fantasy fiction, is it also possible for it to carry the meaning of "magical forces" ("magical forces" in this case meaning an organized body of military personnel which consist of magical beings). I know 勢力 sometimes can refer to an organized body of military personnel, so I was wondering if 魔力 could also function as a fantasy equivalent to 勢力.
It cannot. I mean, if this is a novel we're talking about, you could tell the readers that this is what the word means in the novel's context, but it will be fairly awkward for any reader to get used to.
The word is generally used for the equivalent of mana or magical prowess.
勢力 really refers to the power or influence of something. Here is the definition from Daijisen:
１ 他をおさえ、支配下におくいきおいと力。特に、国家や政党などの社会的な集団がもつ、他の集団をおさえる力。「勢力を伸ばす」「勢力が衰える」 ２ エネルギーの旧称。
- The power or influence to suppress something else and keep it under control. Especially, it refers to the power one societal group (such as a country/state or political party) has to suppress another group.
- The old name for "energy". (e.g. as in the law of conservation of energy)
So, according to this definition, it can't refer to an army, however it could refer to the government or the nation's authority/power. (About the second meaning, エネルギー is the more commonly used word nowadays. So if you see this word, it could also be a synonym of エネルギー.) Yet, it doesn't refer to an army. The same goes for 魔力. Let's look at the definition:
The miraculous/mysterious power to bewilder/seduce or charm people.
This is the more traditional idea of magic power, such as that of an enchantress or enticing demon. Of course, nowadays it takes on many new forms in novels, such as casting spells, etc. However, it does not refer to an army. If you want to say "magic army/forces", here are some of my suggestions:
- 魔法軍 - まほうぐん - magic forces (2000 hits on Google)
- 魔法使い軍 - まほうつかいぐん - magician/wizard forces (4000 hits on Google)
- 魔王軍 - まおうぐん - the devil's forces / Satan's army
To refer to the forces themselves, you can use the word 軍（ぐん） or 軍隊 (ぐんたい). Here is an example of something written by a native speaker that uses these words: https://www21.atwiki.jp/kyokugenmaho/sp/pages/54.html
These get some hits on Google. I'm not a native speaker, so if any native speakers know a better way to say this then feel free to add it.