The book cover is a bit off putting to say the least and the synopsis is written in English, but is a bit bland and boring. A little nitpick on your choice of words here, 'One day, many great beings of other races started a mass murder.' if i'm understanding correctly these other races are mass murdering another race and we have a much more elegant word for it called genocide. I would recommend using that word since its a much more powerful word and it truly conveys the actions you are trying to describe. Of course if you want to pad the word count and use as many words as possible then its up to you, but in my opinion quality is much more important than quantity in writing.
That's my review of your synopsis. 5.5/10 Knows how to use English to assemble sentences.