Even though his actual name isn't used in the book, Antiochus IV Epiphanes is the big enemy, the main villain. The sniveling satraps and government officials don't even compare. Whereas Nebuchadnezzar winds up acknowledging God and prospering, and Belshazzar is doomed by his own stupidity, Antiochus IV Epiphanes is a true egomaniac, a deranged nut who apparently thinks that he himself is divine.
Belshazzar and Nebuchadnezzar were at least devotees of the Babylonian gods, but Antiochus worships no one—except, apparently for a mysterious foreign god of fortresses who he believes can help him in his military campaigns. Like Kanye West, he thinks he's a god himself, and he wants you to hurry up with his croissants.
For Christian readers of the text, Antiochus seemed like a fore-runner of the Anti-Christ—a little taste of what that would be like. Any way you look at, the dude is way arrogant, whether he's appearing as some sort of visionary cartoon horn that can say things, or as an actual person. He's like a little talking tooth from a commercial for toothpaste—riddled with cavities and covered in plaque.
Daniel tells us that Antiochus-horn is going to persecute God's holy ones and wise men, setting up an "abomination that desolates" in the Temple in Jerusalem, itself. This apparently refers to a statue dedicated to Zeus (according to the book of 2 Maccabees 6:1-11), which would sort of contradict what Daniel says about him only worshipping a fortress god. (Maybe in his heart, he could only worship fortress gods.)
Anyway, Antiochus behaves like a big old meanie. We already mentioned persecuting the holy ones, but he also apparently forced many of the Jews to participate in Greek sacrifices, forcing them to abandon their own culture and religion and adopt the ways of the Greeks. That's called cultural imperialism, and it's generally considered to be rather uncool.
So, Antiochus is riding high until the Maccabees' rebellion Macca-beats him. (Pretty good, right?) He limps away from that contest, still alive and still with most of his kingdom intact, but he dies a little while later from disease. Hey, Daniel said he would be struck down, just not by human hands.