This plugin will add a blood moon event to your server, each night has a configurable chance of a blood moon. During a blood mood the game is made a lot harder:

    - Skeletons shoot twice as fast.
    - Skeletons shoot fire arrows (that start fires).
    - Mobs can break blocks when focused on a player.
    - Hostile mobs have double health.
    - Small chance of your sword taking massive damage with each strike.
    - Creepers have much more powerful explosions (that cause fire).
    - Any hostile mob killed has a small chance of coming back as a zombie (or other mob).
    - A mob will spawn if you try to sleep.
    - Spiders can jump much higher.
    - More mobs will spawn.
    - Mobs will drop more XP when killed. 

All of the features can be configured, including the mobs that it is applied to. See the config section for more information.