…or who will speak in the name of other “gods,” that prophet shall die. (Deuteronomy 18:20)
A person may not deliver a prophecy in the name of an idol, such as that it demands our worship. (One may likewise not claim that God instructed a certain idol to be served.)
This is one of those cases where the punishment for violation is explicit in the Torah but one must look deeply to see where the prohibition is stated. The punishment is explicit in Deuteronomy 18:20 (“one who speaks in the name of other ‘gods,’ that ‘prophet’ shall die”). The prohibition presumably appears in Exodus 23:13, “do not mention the names of other gods.” Since we counted this as a separate prohibition in Mitzvah #86, that would make this verse a source of several prohibitions, which is different from a lav she’b’klalos (a prohibition that covers several different things).
This mitzvah applies whenever someone claims to be conveying a message dictated by an idol, even if the thing he is conveying happens to be in accordance with Jewish law.
The reason for this mitzvah is apparent: not only is idolatry anathema to God, posing as an idolatrous prophet misleads people and causes them to stray from the real God.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Sanhedrin (89a-b) and codified in the Mishneh Torah in the fifth chapter of Hilchos Avodas Kochavim. This mitzvah is #26 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #27 of the 194 negative mitzvos that can be observed today in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.