Make the invoke effect of the Tsurugi of Muramasa enhance it to give double damage for a limited time, but also become bloodthirsty during that time. For an immediate effect, it could also cause fear when invoked.
Artifact quarterstaff Staff of Battle. Deals double damage to everything and grants very fast speed when wielded.
To be more true to its source material, Grayswandir should deal double damage only to crossaligned targets rather than all targets (though this would not likely have a huge effect since most wielders of it are lawful and few monsters are lawful).
The blast from an artifact is doubled if the artifact deals double damage to your current form (e.g. if Ogresmasher blasts you while you’re polymorphed into an ogre, the blast damage is doubled).
If your character is male, a small percentage of AT_KICK attacks deal double damage to you.
New artifact Fishing Pole, some sort of polearm that deals double damage to all sea monsters and can hit squares as if you were Expert (in the full 2-square-wide ring around you), regardless of your polearm skill.
New artifact The Captain’s Hook, artifact grappling hook that never fails to do what you want it to do and has zero chance of hooking yourself, regardless of skill. Deals double damage to crocodiles.
New artifact Sinsword, a chaotic two-handed sword. If your alignment is positive, it behaves just like a regular two-handed sword. If negative, it gains a bonus to to-hit and damage (capped at 6) based on how negative your alignment is. Deals double damage to angelic beings.
The Staff of Necromancy: an artifact quarterstaff that deals double damage to non-drain-resistant creatures (but doesn’t life-drain them as the Staff of Aesculapius does), grants cold resistance and curse resistance when wielded, and can be invoked to raise one vertebrate corpse on your square as a tame skeleton.
In cases where Vorpal Blade won’t decapitate because the monster is headless, it does double damage instead with the message “Vorpal Blade goes snicker-snack on the foo!”
New artifact Imaginary Widget: The base object is randomized among non-polearm melee weapons each game. Deals double damage to monsters that don’t exist in real life (e.g. foxes and mastodons and giant eels are real; wargs and mumaks and krakens are not).
Artifact flail or short sword, the Debugger. Deals double damage to all a-class, s-class, and x-class monsters.
New artifact: The Trident of Poseidon. Chaotic (only because Poseidon is in-game). While carried, grants magical breathing, swimming, and protects items from water damage. When wielded, confers water walking. Double damage versus aquatic monsters, and gets a damage bonus versus all that scales with the amount of water squares adjacent to you. Invoke to get an UnNetHack-style flood.
Wands of striking and digging tools deal double damage to all stony monsters like earth elementals, clay golems, stone golems, xorns, and gargoyles.
