The 186th mitzvah is that we are commanded to execute every last inhabitant of a city which has been led to worship idols,1 and to burn the city and everything in it. This is the law of an "ir ha'nidachas."
The source of this commandment is G‑d's statement (exalted be He),2 "And you shall burn the city along with all its goods with fire."
The details of this mitzvah have been explained in tractate Sanhedrin.3
