Why do some astrologers' prophecies come true even though they are liars, and is it true that Imam Ali (may Allah be pleased with him) refuted an astrologer who advised him on a specific time for battle, and he still emerged victorious despite that?

The Prophet, peace and blessings be upon him, informs us that the truthfulness of soothsayers and diviners at times is due to their devils listening to news from the heavens. When they hear a word of truth from the angels, they cast it into the ear of the soothsayer, then mix with it a hundred lies. What proves true from their speech is the word they heard from the heavens before the meteoric projectile overtook them.

As for the account involving Ali, may Allah be pleased with him, and the astrologer, its chain of narration is weak and not authentic; Ibn al-Atheer and Ibn Kathir mentioned it without a chain of narration. The summary of the account is that an astrologer advised Ali on a time to march to fight the Kharijites, but Ali defied him and marched at a different time, refuting the astrologer, and victory was his ally. Ali said: "I wanted to show the people the error of astrologers." Sheikh al-Islam Ibn Taymiyyah mentioned it in a similar vein, stating that Ali said: "Rather, we will travel trusting in Allah and relying on Allah, and in refutation of you."