Rezin was king of Aram-Damascus who allied with Pekah of Israel against Judah during the reign of Ahaz and was ultimately killed by Assyria.
Biography
Second Kings and Isaiah describe Rezin and Pekah attacking Judah and seeking to replace Ahaz with another ruler. Their threat terrified Ahaz and Judah, but Isaiah announced that their plan would not stand and gave the Immanuel sign in that historical setting. Ahaz appealed to Tiglath-pileser III of Assyria for help. Assyria captured Damascus and killed Rezin.
Biblical Significance
Rezin is central to the Syro-Ephraimite crisis that forms the historical setting of Isaiah 7. His failed coalition demonstrates the contrast between trusting God's word and seeking security through imperial alliances.