Albert Barnes Commentary 2 Kings 20:18

"And of thy sons that shall issue from thee, whom thou shalt beget, shall they take away; and they shall be eunuchs in the palace of the king of Babylon." — 2 Kings 20:18 (ASV)

This prophecy had two fulfillments, each complementary to the other. Manasseh, Hezekiah’s actual son, was “carried to Babylon” (2 Chronicles 33:11), but did not become a eunuch in the palace. Daniel and others—not his actual sons, but of the royal seed (Daniel 1:3) and therefore Hezekiah’s descendants—are thought by some to have literally fulfilled the latter part of the prophecy, being eunuchs in the palace of Nebuchadnezzar.