Summon nasties

Summon nasties is a monster spell, able to be cast by level 16 or higher spell-casting monsters.

If you are in Gehennom, there is a 1/10th chance that a demon is summoned. Otherwise, nasties will be generated around the caster until a random number between one and one-third your experience level of nasties that are either neutral or of the caster's alignment are generated. Summoned spellcasters will have a lower level than the caster.

Monsters summoned in this fashion respect genocide, but not extinction.

Strategy

The best way to counter the summon nasties spell is with a ring of conflict. The monsters will fight each other instead of focusing on you. Genocide is a bad idea, since then the summoned nasties will more often be strong, non-genocidable monsters. If you are unable to kill the caster, e.g. an arch-lich that teleports to the staircase across the map on a no-teleport level, and possess reflection and a permanent Elbereth, it is possible to wait on the Elbereth square for the caster to create a black dragon and wait for the distintegration breath to destroy the summoner. This can be a real life saver if any of the Demon Princes appear as well. The main danger is a summoned Minotaur, as they hit hard and do not respect Elbereth.

Messages

"Monsters appear from nowhere!"

1 to (1/3 of your level) monsters are summoned. The number is rounded down to a minimum of 1.

"Destroy the thief, my pets!"

The Wizard of Yendor cast the spell.