This is one of the new wands introducded in Slash'EM. When you zapped, it creates 6-12 monsters. When a monster zaps it, creates 11-14 monsters.

Be careful trying to identify wands as engraving with the wand of create horde wand has the same effect as zapping. You identify the wand if any of the new monsters are seen. Try as hard as you can to keep these wands out of the hands of intelligent monsters; it is quite feasible for such a monster to fill most the level with monsters that way, creating upwards of 200 monsters or so. In addition, they are worth keeping as they, combined with a burned Elbereth square, can be quite useful for sacrificing.

Breaking wand create explosion 1 to (8*charges) damage and create monsters around you.

information from: http://www.statslab.cam.ac.uk/~eva/slashem/wands.html