Rezin was king of Syria, contemporary with Pekah son of Remaliah, king of Israel. In the days of Ahaz, king of Judah, the two kings went up together toward Jerusalem to war against it but could not prevail (2 Nephi 17:1). Isaiah names Rezin’s “fierce anger” against Judah among the threats Ahaz was warned not to fear, calling Rezin and Pekah “two tails of these smoking firebrands” (2 Nephi 17:4). Rezin is identified as the head of Damascus, the capital of Syria (2 Nephi 17:8).
Judah turned to Assyria for help against the Syrian-Israelite alliance. Tiglath-pileser I, king of Assyria, suppressed the aggression, conquered Damascus, and killed Rezin, ending his reign. This is recorded in the biblical and historical record but not recounted further in the Nephite text.
Isaiah’s prophecies refer to Rezin again: the people who reject the waters of Shiloah “rejoice in Rezin and Remaliah’s son” (2 Nephi 18:6), and the Lord declares he will set up Rezin’s adversaries against him (2 Nephi 19:11).