The Bloodmoon mod adds an eerie and difficult new feature to Minecraft, The Bloodmoon strikes randomly but luckily you will get a brief warning ahead of time to prepare. It is a great way to add some tension to your game.
Blood Moon mod adds a lunar event called “Bloodmoon”, every night there is a chance of 5% (configurable) that a bloodmoon will rise. During the following night monsters will spawn faster and closer to the player. The spawn limit of monsters will also be higher. Notice that all of these things only apply to monsters spawned on the surface, you will not experience any of these effects if you are for example in a cave.
You can also not sleep through a bloodmoon night (configurable). You can also turn on an option in the config file to make all monsters spawned by a bloodmoon vanish at dawn (Disabled by default). On the visual side the moon, the sky and general light will be tinted red, to accommodate for the red sky the fog will also turn black for a nicer effect.