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. If a genocided monster would be summoned, it is replaced by an ordinary random monster.

Strategy

One way to counter the summon nasties spell is with a ring of conflict. The monsters will fight each other instead of focusing on you. A scroll of taming or spell of charm monster are also effective, although many nasties have a substantial chance to resist either. Particularly-threatening species, such as minotaurs and master mind flayers, can be genocided, but couatl, fire elementals, and iron golems are all nasties which cannot be genocided. They will be summoned more often if you genocide the other kinds, and a multiple-couatl summon over water could make it very difficult to escape drowning.

If you are unable to kill the caster, e.g. an arch-lich that warps 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 disintegration breath to destroy the summoner, or possibly for a cockatrice to be generated and leave a corpse. This can be a real life saver if any of the demon princes appear as well.

Messages

"A monster appears from nowhere!"

One monster was summoned.

"Monsters appear from nowhere!"

Multiple monsters were summoned.

"<A monster appears/Monsters appear> around <a spot near you/your displaced image>!"

The summoner was fooled by your invisibility/displacement. The summoned monsters may still be able to find you.

"Destroy the thief, my pet(s)!"

The Wizard of Yendor cast the spell while alive. (The Summon Nasties effect that the Wizard uses to harass you in his absence displays no message.)