Then, I visited the idea of human has no inherent mana, and can only make use of the surrounding environment to power the spell.
I'm aware that you use comma to separate three things in a series, but is using a comma here is grammatically correct?
Edit: I think it may be not correct because it only has two things in a series, thus only "and" should be used and "comma" will be redundant (or incorrect altogether)
Yes, commas are used to separate items in a list. But they also have other uses, one of which is to separate clauses, like here.
When there are only two items in a list, we do not normally use a comma. We'd write, "I'm eating an apple and an orange." But if the text of the items is long, we DO sometimes use a comma. "I am preparing a report on the total revenue received by the Eastern region for fiscal year 2016, and on the costs of production that went into producing the products and services that generated that revenue."
When there are two clauses, the comma is optional, and again tends to depend on the length of the clauses and how they flow.
Even if there are only two things in a sentence, it's fine to separate them with a comma (along with some conjunction like "and", "but", "or", etc.). Some examples:
In this sentence I use a comma, and I use a conjunction.
It's fine to use the comma to indicate a separate thought, or a natural pause if the sentence was spoken.
