Adding onto Ringil's post.
The crux of the question
what I do not understand is, what kind of form is 「強から」.
To understand this, we need to look more deeply into how い adjectives can be inflected or conjugated.
Some background: inflected forms of い adjectives
As Ringil notes, this derives from base adjective 強【つよ】し (classical terminal [sentence-ending] form), modern 強【つよ】い.
In Classical Japanese, the い adjectives had two inflection patterns. One of these evolved into the modern one you're probably familiar with, and is generally called the 本【ほん】活用【かつよう】 or "main" conjugation.
The "main" conjugation pattern for Classical い adjectives: ク活用【かつよう】
Another name for this pattern is ク活用【かつよう】, so-named for the adverbial form ending in く. Japanese grammars distinguish this from the シク活用【かつよう】, used for those い adjectives that end in しい, like 美【うつく】しい or 素晴【すば】らしい. (We won't go into the シク活用 pattern here.)
- 強し - the 終止形【しゅうしけい】 or terminal form, used to end a sentence or clause
- 強き - the 連体形【れんたいけい】 or attributive form, used to modify a noun
→ As Classical evolved into modern, the attributive lost the //-k-// to end in just //-i//, and then the attributive replaced the terminal.
- 強く - the 連用形【れんようけい】 or adverbial form, used to modify a verb or clause, or otherwise indicate the manner
- School grammars also list the 未然形【みぜんけい】 or irrealis ("hasn't happened yet, or hasn't completed yet") form, mostly used for the negative. For い adjectives, this is the same as the adverbial.
- 強けれ - the 已然形【いぜんけい】 or realis ("has happened already") form, most commonly used in modern Japanese for the conditional ば
- There isn't any 命令形【めいれいけい】 or imperative form for い adjectives.
The "other" conjugation pattern for Classical い adjectives: カリ活用【かつよう】
This other inflection pattern has mostly disappeared from the modern language, outside of some set phrases. This pattern is called the カリ活用【かつよう】 or "kari" conjugation.
- There isn't any 終止形【しゅうしけい】 or terminal form
- 強かる - the 連体形【れんたいけい】 or attributive form, used to modify a noun
- 強かり - the 連用形【れんようけい】 or adverbial form, used to modify a verb or clause, or otherwise indicate the manner
- 強から - the 未然形【みぜんけい】 or irrealis ("hasn't happened yet, or hasn't completed yet") form, mostly used with the negative and certain other auxiliaries
- There isn't any 已然形【いぜんけい】 or realis form for the kari pattern.
- 強かれ - the 命令形【めいれいけい】 or imperative form
This kari conjugation is actually a contraction, formed from the regular く adverbial ending + the various inflections of あり (the Classical "to be" verb, as found in modern あります). This is also where we get the modern "past tense" for い adjectives -- forms like 強かった are contractions of 強く + あった.
Your problem word: 強からん
As we see above in the kari conjugation pattern, 強から is the 未然形【みぜんけい】 or irrealis stem used with the negative and certain auxiliaries. This 強から is essentially 強く + あら, where あら in turn is the 未然形【みぜんけい】 or irrealis stem of あり (modern ある).
The ん on the end is a contraction, in this case, of the suppositional / volitional auxliary む. This attaches to the 未然形. Over time, this //-amu// ending became the modern suppositional / volitional ending //-oː//.
(I've delved into these historical sound changes in む in a different post; please read that for more detail.)
So 強からん = 強からむ = 強く + あらむ = (in modern Japanese) 強いだろう.
Side note: 強か
「強か」 is a な-adjective
In that case, the reading is したたか. While a valid reading, that form doesn't fit the grammar of the sentence, since then we'd have 強【したた】からん, and there is no らん or らむ ending that attaches straight onto the root of な adjectives.