Where abouts in the Bible is the great monarch?
The most obvious place is in Rev.6:2 - the man on the white horse. He conquers the forces opposed to Christianity during the next World War. The scroll is the legal basis for the Kingdom of God on earth and is opened bit by bit over a period of time. The white color refers to the legal right to practice religion according to one’s conscience without oppression or persecution.
Nothing in the old testament?
Only very indirectly - Zech.1:8-11 for example. The Great Monarch is Catholic, not Jewish,
so the Old Testament doesn’t address him directly; also it doesn’t mention the three world wars, except by analogy in Judges19-20.
I never heard any Catholic teaching about a great monarch and three world wars except by fringe "Apologists" who seem to know exactly when and how the world will end. Most of it is based on Evangelical tribulationism, personal literalist interpretation of the Book of Revelation in light of current events, or some mixture thereof.
The reign of the Great Catholic Monarch is always prophesied to be "imminent", and when that doesn't happen it becomes "within our lifetime" and when that doesn't happen it is changed to "sooner rather than later." It always seems to end in a webpage which predicted that the apocalypse would happen in 2008 or 2009 or 2010 or 2011 or
What is never explained is how monks meditating on Scripture two hours every day for 1600 years were able to miss all this. :shrug: