Albert Barnes on Revelation 19:12

His eyes were as a flame of fire. Rev 1:14.

And on his head were many crowns. Many diadems, indicative of his universal reign. It is not said how these were worn or arranged on his head--perhaps the various diadems worn by kings were in some way wreathed into one.

And he had a name written. That is, probably on the frontlet of this compound diadem. Compare Rev 13:1; Rev 14:1.

That no man knew but he himself. Rev 2:17. This cannot here mean that no one could read the name, but the idea is, that no one but himself could fully understand its import. It involved a depth of meaning, and a degree of sacredness, and a relation to the Father, which he alone could apprehend in its true import. This is true of the name here designated--"the word of God"--the Logos--λογος; and it is true of all the names which he bears. See Mt 11:27. Compare a quotation from Dr. Buchanan in the Asiatic Researches, vol. i., vi. p. 264, as quoted by Rosenmuller, Morgenland, in loc.