If you like to make harder the survival mode of Minecraft this mod may interest you.
The You Will Die 1.7.2 is a mod that aims to make life much more complicated than it normally is in Minecraft. At the begining, when creating a new world, the world will have a blessing but also a curse, so you will find things that will bother you and others that will benefit you.

Detail and Download here : The You Will Die Mod

Homepage: Mods for Minecraft