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. Spellcasting monsters generated this way need not be lower in level than the caster.

Strategy

Dealing with this spell can require actions both before and after it is cast:

Preventative genociding

One option is to genocide the worst nasties: master mind flayers and master mind flayers are good candidates for this. One should be cautious, however, in doing so. If the spell attempts to generate a genocided monster, it will instead generate a random one. Once the player has the Amulet of Yendor and is out of Gehennom, Archons and ki-rin are not unlikely random monster choices.

A better option for a player with a scroll of genocide would be to blessed-genocide L. Liches are not only the most frequently encountered casters of summon nasties, they are also the only ones which can be genocided anyway. Doing so will genocide only one nasty, arch-liches, and so will not greatly increase the probability of Archons being generated by the spell.

When meeting a summoner

The worst place to meet monsters with this spell is on large open levels, e.g. Juiblex's swamp. On such levels, a summoner will often sit outside of a ring of monsters it has created around you, and continue summoning even as you attempt to fight off its minions. In this respect the covetous behavior can be helpful: an arch-lich that warps to the upstairs may no longer be in sight, whereas a fleeing demilich will remain in sight and happily fill the level with nasties.

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.

Another 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.

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.)