One may not consult a necromancer, that is, one who supposedly channels the dead. This type of sorcerer appears to make disembodied voices answer questions, so it probably involved some form of ventriloquism.
Once again, the reason for this prohibition is that it tricks people and causes them to stray from God after smoke and mirrors – sometimes literally!
This mitzvah applies in all times and places. In the Talmud, it is discussed in tractate Sanhedrin on pages 65a-b; it is codified in the Shulchan Aruch in Yoreh Deah 179. This mitzvah is #36 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #172 of the 194 negative mitzvos that can be observed today in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.