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.