The ruler of militant state decides to make war against the peaceful Flavestina and conquer it. Relying on his army, he doesn’t at once accept his Astrologer’s advice: to use magic for sure victory. The magic ritual, however, will require stealing the Golden Cup from the altar of the Temple of the Sun in Flavestina. Army and magic against love and courage: who will win?

What kind of ruler is he? Inexperienced, young, headstrong, etc...

What does he actually do in the story? Your first 3 sentences describe the setup of the movie (2 sentences too long), and you never actually describe what happens, what the ruler's main 'quest' is. Where does "love" enter the picture?

You could narrow that setup down to half a sentence- "Convinced by his top advisors to invade a neighboring peaceful country, an inexperienced ruler...[WHAT HE DOES]." Something like that-

Your logline is simply too long; 50 words should be your ultimate limit, and closer to only 25 words would be much better.

Also, logline readers don't have time to learn your whole story world's mythology, so trim out all but the "nugget" of the story.

For example I suspect your logline is leaving out the pivotal relationship in the story. Is the story's core relationship really that between the ruler and his astrologer? Or does Flavestina have a ruler (like a princess) who will actually become the most important influence in the militant ruler's life?

Basically, you're so fixated on all the story-world stuff here, you don't set up a relationship that pulls us in. See if you can find the successful loglines for "The Princess Bride," "Ever After," "Labyrinth," "The Lord of the Rings," etc.