Define some monsters (not necessarily all monsters) as diurnal or nocturnal. During the daytime, monster generation biases slightly towards diurnal monsters and away from nocturnal ones, and nocturnal ones may generate asleep when they do generate. At night, this reverses.

A challenge game mode called “Midnight Run” in which the time of day according to the game is permanently locked to midnight.

Add the following events on Halloween:

• The normal “midnight” effects should begin at 6 PM.
• Intelligent monsters may generate peaceful and with candy bars, which they will give to you when you chat to them. Or possibly, this only happens with monsters that have “homes” such as shopkeepers, priests, and peaceful Minetown inhabitants. (Not specified what happens if a monster generates with a candy bar, but you save the game and come back to talk to them once it’s no longer Halloween.)
• Other monsters have a higher than normal chance of generating with attack wands.

Draugr (and any undead playable race) should deal double damage at midnight.

Artifact sword Galatine, which is the sword of Gawaine in Arthurian stories. It has a damage increase depending on the time of day, after how Gawain’s strength waxed until the hour of noon and faded afterwards. +1 from 9 to 10 AM, then +2, +3, +3, +2, +1, +0, -1, -2, -1. Then from 7 PM to 9 AM, +0.

Lava gremlins: a different gremlin species that reproduces in lava instead of water. It has at least one physical damage attack and at least one fire attack, and the regular gremlin intrinsic-stealing attack, except they steal intrinsics during any time that is not night.