Monster Swarm Mod empowers monsters such as zombies and skeletons, makes them even more threatening and swarmy. The Al of Minecraft mobs are instensified by this mod. Monsters now can attact you from even as far as 120 meters away and they can kill your animals. The monsers also have the ability to build bridge as well as destroy walls which lock their way. However, depends on the material of those walls, the process might be slow. And be careful! They can also attact your base when the night falls down. Generally, this mod makes bricks more immune and obsidian is nerfed as it changes the blash-resistance of some blocks. Config file is pretty basic, but self explanatory. You can use commmand /swarm reload to reload.