The Essence
Maalik-ul-Mulk is the absolute Owner of the entire kingdom of existence. He does whatever He wishes within His kingdom, giving dominion to whom He wills and taking it away from whom He wills.
In the Quran
قُلِ اللَّهُمَّ مَالِكَ الْمُلْكِ تُؤْتِي الْمُلْكَ مَنْ تَشَاءُ
"Say, 'O Allah, Owner of Sovereignty (Maalik-ul-Mulk), You give sovereignty to whom You will...' (Surah Ali 'Imran 3:26)"
In Your Duas
اللَّهُمَّ مَالِكَ الْمُلْكِ تُؤْتِي الْمُلْكَ مَنْ تَشَاءُ
"O Allah, Owner of Sovereignty, You give sovereignty to whom You will..."
— Quran, Surah Ali 'Imran 3:26