A helm of darkness artifact, inspired by the one from Perseus’ myth; when worn it confers invisibility and produces darkness in a radius around the wearer.

Rogues gain a backstab-related bonus when they and their target are in darkness. Possibly, just make many/most attacks in darkness deal backstab damage when they ordinarily wouldn’t.

A more complex implementation would be to differentiate monsters that aren’t susceptible to darkness because they spawned in a naturally dark area from monsters that are.

New artifact “Darkenstone”, which acts like the Arkenstone except that it’s a black opal, and carrying it provides radius 2 darkness. If drow are in the game, this is often given to them as a gift.

Black lights have a radius 1 area of darkness instead of light.

Darkness elemental: darkens squares it moves through (removing the lit state, if the square was permanently lit beforehand).

More harassment effects: the chance of respawning the Wizard is a fixed 1/7 and the rest of the effects are equal probability. Multiple non-duplicate effects may happen at the same time.

• Unchanged: curse items, summon nasties, vaguely nervous, aggravate monsters
• Darken much of the level, and extinguish, destroy, or cut the remaining lifetime of light sources. Maybe also blind the player.
• Drop boulders around the upstairs, or iron bars.
• Spawn pools centered on the player, with sea monsters in them.
• Spawn lava around the level, not under the player.
• Create a hole or pit under the player.
• Create a stinking cloud centered on the player.
• If you are carrying the Amulet, it teleports to a random square on your level or the level below.
• If you are carrying the Amulet, it (and possibly any fake Amulets) becomes heavier. This either lasts until the next harassment triggers, or is permanent but the total amount of weight added to the amulet has some sort of cap on it. This is interesting primarily because it targets carry cap, which is a valuable end-game resource.
• A variety of powerful player debuffs such as dealing half damage, taking double damage, blocking HP or Pw regeneration, super hunger, etc.