The Banes are a group of artifact weapons, notable in that they do double damage to a specific group of monsters. They are generally considered to be worthless for most purposes, since they reduce the probability of receiving a more useful artifact. Despite its name, Magicbane is not included since it has no double damage bonus. Wolfsbane is not a weapon at all but a comestible, named after a real-life plant. Likewise, Fire and Frost Brand are not included, as their range of speciality is very large, while Sunsword has several other abilities as well.
In UnNetHack and DNethack all these artifact weapons (with the exception of Sunsword) also give you warning of those monster types they do double damage against. For example, wielding Dragonbane will show you all dragons on the current level.
Demonbane is a lawful long sword that deals double damage to proper demons, and has +1d5 to hit them. It acts as a normal long sword against non-demons, including the Riders, the mail daemon, sandestins, and djinn. Its utility or lack thereof depends on your opinion of the difficulty of most demons. As of version 3.6.1, it now prevents demons from summoning help.
In dNethack, it is now a lawful silver saber, and blocks passive summoning abilities of demons, even when wielded in the offhand. Its double damage and warning are preserved. In SporkHack, it is now a lawful silver mace, and blocks summoning abilities as dNethack. It is also the first gift for SporkHack priests.
In dNethack, it is a lawful lance that cannot break, and grants its bonus also to any reptilians. In SporkHack, UnNetHack, and SLASH'EM, Dragonbane is instead a dwarvish spear that gives all dragon breath resistances (fire, cold, shock, sleep, poison, acid, and disintegration) when wielded. It also has a 50% chance of instantly killing dragons (reduced to 20% in UnNetHack).
Giantslayer is a neutral long sword that has +1d5 to hit and does double damage against proper giants, which include giant mummies, giant zombies, and even Cyclops and Lord Surtur, but not ettins or titans.
In UnNetHack it is now a neutral sling, and sets the player's strength to 18/** when wielded or in the off-hand. Note that slings also get a larger strength bonus in UnNetHack, so this might be a good choice of weapon for a player that likes slings.
In SLASH'EM, this is replaced by Giantkiller, a neutral axe. It has mostly the same stats, but has an added 15% to chance any giants (as listed above).
Ogresmasher is an unaligned war hammer that confers +1d5 to hit and double damage against ogres. In dNethack, Sporkhack, UnNetHack, and the vanilla 3.6.1 development version, it also sets the wielder's constituition to 25 when wielded or unused in the off hand. This makes it slightly more useful.
Sting is a chaotic elven dagger that provides +1d5 to hit, and does double damage to orcs, including orc mummies and zombies. It also reveals the location of orcs on the level, and cuts through spider-webs. Any player can obtain it readily by #naming any elven dagger "Sting," provided Sting does not already exist.
In dNethack, it also grants warning vs arachnids (any 's'), in addition to the standard double damage.
Sunsword is a lawful long sword that does double damage to all undead creatures: L (liches), M (mummies), W (wraiths, wights and Nazgul), Z (zombies and skeletons), ghosts, and shades. It is made slightly more useful by the fact that it's a light source, with a radius of 2, and that it prevents blinding from light-based sources. Also, its damage range is somewhat wider and more useful than that of the other Banes, especially since many spellcasters are undead. The keep-or-toss decision is based on how much you hate undead.
In dNethack, it is considerably more useful. Many new and powerful undead are around, particularly spellcasters. In addition, it now grants 2x damage versus demons, and petrifies trolls and gremlins. It may also blind struck enemies. Lastly, it now uses (and trains) the higher skill of short swords and long swords, which makes it effective even for, say, a rogue, and maybe even a wizard.
In dNethack, it also grants 2x damage and warning against demons, which makes it slightly more useful (though still more useless).
Trollsbane is an unaligned morning star that does double damage and +1d5 to-hit to all T (trolls). In the nethack 3.6.1 dev version, it also prevents trolls from reviving while wielded. In SporkHack and UnNetHack, it grants hungerless regeneration and a chance of instantly killing trolls.
In dNethack, it is now silvered and instantly kills trolls. In addition, it grants extra damage vs regenerating and covetous monsters, and petrifies gremlins.
Werebane is an unaligned silver saber. It does double damage against all werecreatures, as well as +1d5 to hit. However, it is slightly more useful than the other banes in that when wielded, it protects you from lycanthropy. It also has a useful base item, and therefore may be more useful than another bane.
In dNethack, it also grants warning versus demihumans, which include, in rough order of rarity:
- all nagas (including hatchlings)
- centaurs (but not sprow)
- deep/deeper/deepest ones
- (master) mind flayers
- gnolls (matriarchs and ghouls included)
- githyanki pirates
- (warrior) changed
In addition, it grants warning versus the following 'boss' monsters:
- Elder brains (Anachronaut quest bosses)
- Agalope (Bard quest nemesis)
dNethack adds more banes, ranging from moderately useful to highly useless.
- Nodensfork: an unaligned silver trident that grants slotless warning and extra damage against primordials (elementals and similar), aliens (mind flayers, U (unknown abominations), and a couple drow) and telepathic monsters.
- Gaia's Fate: an unaligned sickle that grants slotless warning and extra damage against plants, insects, arachnids, avians, reptilians, animals, fey, elves, and elementals. Because of its long warning list, it's absolutely worth keeping around in open inventory.
- Vampire Killer: a lawful bullwhip that grants extra damage to all, grants drain resistance while wielded, and slotless warning against undead, demons, and werecreatures. You can also #invoke it to blessed, fix and erodeproof, and set enchantment to +3 if +2 or lower.
- Kingslayer: a chaotic stilleto that is permanently poisoned, grants slotless warning against monster lords or princes (including the late-game demon and devil lords), and can be held second to other artifacts with no penalty or restrictions.
- Peace Keeper: a lawful athame which grants extra damage and warning vs always hostile creatures. It can be held second to other artifacts with no penalty or restrictions.
In addition, two more new nameable 'banes' exist:
- Claideamh: unaligned long sword which warns and grants extra damage against elves, fey, giants, elementals, and primordials.
- Carnwennan:: lawful dagger that grants +1d5 to hit, +1d10 damage, and warning vs magic-hording monsters. Can only be named by knights.
- Grimtooth: chaotic orcish dagger (as in vanilla), now grants +1d5 to hit, 2x damage, and warning against elves, humans, and minions.
Lastly, there is another bane for werecreatures, but it's very useless. Werebuster is an unaligned long sword with +1d10 to hit and +1d20 to damage against all werebeasts. It is guaranteed at the Chaos quest, but likely isn't even worth picking up.
Some non-artifacts with similar names: