A cataclysm is an event that causes great change or harm. a violent or sudden event or change, such as a serious flood or earthquake (cataclysme, from , from , from kataklyzein , from kata- + klyzein )
A cataclysmic event is one that changes a situation or society very greatly, especially in an unpleasant way. Few had expected that change to be as cataclysmic as it turned out to be
[ 'ka-t&-"kli-z&m ] (noun.) 1637. French cataclysme, from Latinized Greek cataclysmos, from Ancient Greek κατακλυσμός (kataklysmos, “deluge, flood”), from κατακλύζω (kataklyzo, “to dash over, flood, deluge, inundate”), from κατά (kata, “downwards, towards”) + κλύζω (klyzo, “to wash off, to wash away, to dash over”).