"It's dark there, that's why I think that he is going to fall."
Translation:Там темно, поэтому я думаю, что он упадёт.
It is sound unnatural out of context, because in this order you put unexpected stress on the word "темно". It might make sense in a conversation tho. For example: "Почему ты думаешь, что он упадёт?" (Why do you think that he's going to fall?) "Темно там, поэтому и думаю!" (It's dark in there, that's why!)
TL;DR: "он упадёт" means completed action in future, "он будет падать" means continuous action in future. Verb "будет" used with imperfective verbs in future tenses.
In the Russian language, there are different verbs for perfective or imperfective aspect. This feature serves roughly (!) the same purpose as perfect tenses in the English language. For example,
to fall - падать (imperfective), упасть (perfective);
to suffer, to expirience hardship - страдать (imperfective), пострадать (perfective);
to provide light - светить (imperfective), посветить (perfective).
Now, things are actually a bit more difficult, because there are usually two verbs of imperfective aspect: determinate and indeterminate (not in examples above). Also, one verb of imperfective aspect (for example, плыть, swim) might have several different corresponding perfective aspect verbs (уплыть, приплыть, заплыть, etc) often with a different meaning, which in their turn each have corresponding imperfective aspect verbs (уплывать, приплывать, заплывать, etc). Don't think about it. Russians never do.
Perfective verbs only have past and future tense forms. Don't think about it too much either, after a lot of practice it will be intuitive. The important thing is that imperfective verbs don't have the future tense form, so to compose a future tense with an imperfective verb you need to use "быть" in future tense form + verb infinitive.
Она будет смеяться (She will be laughing)
Я буду бегать (I will be running)
Оно будет петь (It will be singing)
You do not normally do that for perfective verbs. There are exceptions, but again, don't be bothered at this point. Instead, you just use their future tense forms:
Она засмеётся. (She will start laughing)
Я убегу. (I will run away)
Оно споёт. (It will sing and it will finish singing whatever it will be singing)
So, 'он упадёт' (perfective aspect) is somewhat like 'he will have fallen', meaning that in future he will participate in an act of falling and this act will be concluded, presumably, by a landing of some sort. 'Он будет падать' (imperfective aspect) is like 'he will be falling', meaning that in future he will participate in an act of falling that will not have any specified conclusion and might go on indefinitely or (more likely) that he will repeatedly and relentlessly indulge in the act of falling, getting hight, and falling again.