Which word is the best synonym for the word “foreign” as it used in this sentence? frightening weird upsetting unknown

A synonym is a word that means the same thing or has similar meaning. 
Foreign is defined as strange and/or unfamiliar. Frightening is not defined as strange or unfamiliar. Weird is defined as 'something supernatural'. Upsetting is not something strange or unfamiliar. Unknown is defined as 'not known or familiar'.
Your best synonym word is "Unknown."
I hope this helps!