could this work? I mean, is it giving way too much?

Um, try: I was a Phillisin and the Sapharios were our mortal enemies since before time began, though no one knew why.

alvin123 said:

I sighed, thinking about the reason why the sapharios became mortal enemies with us phillisins.

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Is the character wondering what the reason is, or thinking about the reason, which the character already knows?

Delete either the word "why" or the words "the reason." That will improve it noticeably.

Dale

alvin123 said:

I sighed, thinking about the reason why the sapharios became mortal enemies with us phillisins.

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Does the reader already know the POV char is a Phillisin? If so, you don't need to repeat that information. Also, changing the verb tense (from "became" to "had become") indicates the event of becoming enemies happened sometime previous to when the character is thinking about it.Or is this the beginning, and you're trying to introduce two pieces of information: the POV char is a Phillisin and the Sapharios are enemies of the Phillisins? In that case, it's probably better to break it up. (But even better yet to handle the opening completely differently.)