The 1837 edition introduced an inexplicable intrusive that (“and now that it came to pass that …”). Amazingly, this typo continued for some time in the text. It was finally removed from the LDS text in the 1852 edition and from the RLDS text in the 1908 edition.
Summary: Ignore in Mosiah 18:1 the subordinate conjunction that (before the phrase “it came to pass”) that was accidentally introduced in the 1837 edition.