NK Fusion Bomb
The bomb tested by North Korea had a yield 10-15 times greater than the Hiroshima bomb, so was it a true fusion bomb? We don't know yet, but it seems likely. 120 Kilotons, while on the small side for a fusion bomb, is really big for a fission bomb. One possibility is that it was Sakharov's First Idea type layer cake bomb, a simpler fusion assisted type of bomb which probably can't get either really big yields or fit on a reasonable sized missile. Ulam-Teller type designs, used by all the other thermonuclear powers are able to be both compact and extremely powerful.
In any case, a 120 kiloton bomb can devastate a large city.