What is the doctrine of "Wahdat al-Wujud" (unity of existence)? And what is its ruling?

The doctrine of Wahdat al-Wujud (Unity of Existence) is a false and blasphemous doctrine adopted by some extremist Sufis, such as Ibn Arabi. Its essence is that there is no existent being except Allah, and that the existing phenomena are manifestations of the divine reality, and that the universe, in their claim, is Allah. Ibn Arabi says: "The servant is the Lord, and the Lord is the servant," which indicates his adoption of this doctrine. Evidence has shown its falsehood, for Allah is the Lord of the worlds, and everything other than Him is created, as stated in His saying, the Almighty: "Allah is the Creator of all things."