It’s the Assyrian known by many other names and titles
Nineveh was the capital city of the Assyrian Empire also known as the land of Nimrod Micah 5:6. By default anyone coming out of Nineveh was an Assyrian if they were a native.
But it’s not referring to a human, or the Assyrian people, it’s a spirit. A spirit of wickedness, revolt and rebellion against Elohim. We also know it as the antiChrist spirit.
If you look at the Septuagint in Daniel 10:20 where we read in English the prince of Persia in the LXX its the king of the king. If you understand (even if you don’t ascribe to) the divine council and sons of Elohim which later became the gods of the nations, you’ll realize that behind the king was another king or spirit. Paul calls them principalities, powers, rulerships or daimonions. In like manner this wicked one will inhabit a total of seven persons/kings and one of them he will inhabit/possess twice. Rev 17:10,11
He is the serpent of old, satan, the devil, son of perdition, the wicked one, the Assyrian, Lucifer, son of the morning, king of Tyrus. All the other gods/sons-of-elohim gave their powers to him, conspiring against Elohim after their judgment in Psalm 82
“and said to him, “I will give you all this power and glory. It has been handed over to me, and I can give it to whomever I choose.”
Luke (Luk) 4:6 CJB
When I read Nahum 1:11 I read the spirit of antiChrist or satan. He took on in history 6 or 7 times the role of king of the king in an attempt to destroy the seed lineage to prevent the redemption of humanity. So you can read it purely from a material stand point or you can read it with the spiritual dimension in mind. I choose to read it with the spiritual dimension in mind because the spirit behind the king is the one that is truely pulling the strings.
Also the Assyrian the man or the spirit or both were in Egypt at the time of Israel’s captivity.
“For thus saith the Lord God, My people went down aforetime into Egypt to sojourn there; and the Assyrian oppressed them without cause.”
Isaiah 52:4 KJV
He is not limited to just one geographic location as generally are/were the gods of the nations, but he still carries the title. In part because of the cruelty with which the Assyrians treated their enemies but not limited to. So the Assyrian can be the king of Tyre and the Pharoah/king of Egypt. He doesn’t have to be only in one geographic region or attached to only one people