He built an altar to the god Jupiter.
Antiochus IV was the 8th ruler of the Seleucid empire. He called himself 'the visible god' because he believed that he and Jupiter were identical. The visible refers to the fact that the planet Jupiter can be seen with a naked eye. He identified himself with Jupiter and many people thought him a madman. His biggest enemies were the Jews and he was determined to exterminate them.
In 168 BC, he devastated Jerusalem and put a pig on the Temple's altar there. He then erected an altar to Jupiter, killed many Jews, destroyed temples and copies of Scriptures and put many Jews into slavery. He also tortured many of them in order to renounce their religion.