DeadlierNights is an in-development plugin designed to make nighttime more dangerous for imprudent players.
At present, it allows for potion effects to be applied to players who remain in complete darkness for an excessive amount of time. The delays, effects, and strengths of the debuffs can be specified. Messages can also be sent to the player when the effects are applied or remove.
Planned future features include buffed mob spawns near players who linger in the dark and longer-term effects resulting from staying in the dark for too long. Also planned is a way for the severity of the debuffs to be tied to the phase of the moon and/or the current weather conditions.
Feedback is both welcome and requested- I am still learning the specifics of the API.
There's a forum thread for the mod here.
The project's source is available on GitHub. You'll also find the most recent .jar there, but please be aware that it is not vetted by Bukkit's staff and may be completely unstable!

