John Calvin Reformed @genevareformer

Here Daniel shortly relates how his prophecy was fulfilled that very night. As we have before explained it, a customary feast-day had occurred which the Babylonians celebrated annually, and on this occasion the city was betrayed by two satraps, whom Xenophon calls Gobryas and Gadatas.

Matthew Henry Presbyterian @wholebiblehenry

Here is, 1. The death of the king. Reason enough he had to tremble, for he was just falling into the hands of the king of terrors, Dan 5:30. In that night, when his heart was merry with wine, the besiegers broke into the city, aimed at the palace; there they found the king, and gave him his death's wound.

Commenting on Daniel 5:30-31

John Gill Reformed Baptist @doctorgill

In that night was Belshazzar, the king of the Chaldeans, slain. Not by a servant of his own, as Jacchiades; or by an eunuch, one of his guards, as Saadiah and Joseph ben Gorion (b); but by Gadales and Gobryas, who led Cyrus's army up the river Euphrates into the city of Babylon, its course being turned; the inhabitants of which being revelling and rioting...