A wand of create monster generates monsters around your character, consistent with the monster level your character normally generates.
As with all wands, the BUC status of the wand does not have any effect on the outcome, other than the usual 1% chance of a cursed wand exploding when zapped.
Assuming the wand still has charges, breaking it will create a horde of eight  monsters around the player.
In Rogue, there was no wand of create monster, per se, but a scroll of the same name; it would produce the message "You hear the cries of anguish in the distance." when a monster read the scroll if the player was already surrounded.
The literal wand of create monster made its debut in Jay Fenlason's Hack.
If you want to maximize your sacrifice material, you should break a wand with one charge left, so long as you are able to survive the onslaught and the damage from the magical blast. (Tips: stand on burned Elbereth, be magic resistant, have escape items in case you get e. g. captains or archons.) In a temple, don't catch the priest in the blast or you will anger him.
Breaking generates 18.8 monsters per wand on average if you can identify the number of charges. If you don't know the number of charges, the optimum strategy is to zap it 9 times, then break it for expected 14.0 monsters. Zapping and wresting the last charge produces only 13.2 monsters.
If you want to generate lots of garden-variety monsters without violating any conducts, you should repeatedly #loot a throne while confused and let strong pets relieve the throne monsters of their wands.
Autopickup exceptions are often set to pick up wands of create monster, if only to keep them out of monsters' hands.
Wands comprise 4% of all randomly-generated items in the main dungeon, 6% in containers, 5% on the Rogue level, and 8% in Gehennom. There is a 4.5% chance that a randomly-generated wand will be a wand of create monster. Monsters also have a significant chance of getting one as their offensive item.