Petrification is an effect that instantly kills its target. It can be cast by the PC on a monster, or can be inflicted on the PC by monsters (though at present, only gorgons do this).
Cast by the PC[]
The Petrification spell is usually very difficult for PCs to learn, and has a very high base PP cost (120). When cast, the PC can target one adjacent monster and, if the spell works, the target will not only die instantly but also be turned into a Statue corresponding to the type of the petrified monster. Killing monsters in this manner yields no item drops, although any items they had picked up and/or equipped will be dropped. Killing the Minotaur emperor with petrification will still yield his usual drops. Named monsters, funnily enough, keep their name when turned into a statue (e.g.: hill giant statue of "Bonk"). Non-immune, non-magic-resistant monsters have around a 50% chance to resist petrification. Some monsters, including all monsters that cannot be wished for (excluding monsters with special wish behavior), are completely immune to petrification.
Unlike some spells, the effects of the Petrification spell can not be replicated by any wands.
Cast by an NPC[]
The only monster that can petrify the PC (so far) is a gorgon. Unlike the spell, which can only hit one adjacent square, the petrification gas breathed by a gorgon can reach two or three squares in front of it. The breath might also annihilate any other monsters in its way, should it manage to hit them.
Should the PC be hit by petrification gas, and does not have petrification resistance, (s)he will die instantly. Resistance is usually quite rare to find; it is only granted by an amulet of petrification resistance, by a certain few artifacts, or from pools. It is sometimes possible to dodge gorgon breath, possibly depending on the PC's DV and alertness; though the PC usually shouldn't count on this.
As well as being breathed on, there is another way for the PC to be petrified by a gorgon — eating it. The PC should only ever eat a gorgon if they are completely sure they have petrification resistance.