Slaying is a term used to describe a property that increases a character's accuracy and damage in melee and ranged combat. Slaying does not affect spellcasting, wand use, or anything else that isn't melee or ranged combat.
The total of all of a character's slaying bonuses are added to the player's to-hit number. A random number from 1 to the player's total to-hit is checked against a monster's EV to determine if the player lands a blow; thus, it is not a flat accuracy increase that is provided, but an increase in accuracy on average.
The character's total slaying bonus is added to the "effective enchantment" of the player's weapon (and applies to unarmed combat as well). In combat, the player gets a damage bonus of 1d(effective enchantment). Unlike base damage, this bonus is not affected by the player's skill. The total damage done is then adjusted appropriately by the monster's AC, meaning that slaying can be very useful in helping to get past a monster's armour.
There are a number of sources of slaying bonuses:
- A ring of slaying.
- Artefacts may provide a bonus or penalty, in which case the description says "It affects your accuracy and damage with ranged weapons and melee attacks."
- The spell Song of Slaying provides a variable amount of slaying, depending on the toughest monster you've killed while the spell is active.
- Gloves with archery ego are a special case, in that they provide a +4 slaying bonus but only affect ranged combat.
There are also a number of slaying-like effects:
- The effect Might (which is also a part of Berserk) gives a flat +1d10 bonus to melee damage. This is somewhat different from normal slaying in that it is not added to the character's effective enchantment, but just added straight on to his or her total damage. Effectively, this means that Might gives a more consistent damage bonus than 10 slaying would, even if the average increase in damage is roughly the same.
- The spell Infusion gives a damage bonus of 2-4. However, monster AC applies to this damage bonus separately from the character's main attack, making it more akin to an extra attack with an unenchanted dagger than a slaying bonus.
- The spell Corona gives an increase in accuracy, but only against one monster. Also, the bonus (+2 to +9) is applied after the to-hit number is rolled.
- The Shining One's halo gives an accuracy bonus against creatures inside the halo.
- An amulet of inaccuracy functions somewhat like a -5 accuracy bonus, except that it affects magic as well as melee and ranged combat.
Most characters will want as much slaying as they can get.
Melee characters should highly prioritize slaying, but be rational about it: a +2 slaying bonus does not make a -1 randart scale mail a worthwhile proposition on your MiBe.
Prior to 0.15, there were separate slaying bonuses for accuracy and damage. Bonuses were shown in the form +x, +y, where x was the accuracy bonus, and y was the damage bonus
Prior to 0.10, slaying bonuses for damage were added to the weapon's base damage, meaning that they would be increased by having high weapon/Fighting skill. When this was changed, slaying bonuses were generally increased to compensate.