This is Advanced Darkness. It creates a more interesting and dangerous experience in Minecraft by reducing the overall power and effectiveness of light sources. This is done by enforcing a far lower than normal gamma, reducing your world to pitch black with no lights and slightly less with lights around. Fear not, however, because you can mitigate this effect to an extent by crafting Comforting Lanterns, to be used around your home to brighten up the place. Good luck, and don't forget to carry a light with you!