The idea of this build is to create a supportting Amumu, using his stuns mainly to begin team fights and helping allies to get some kill, always depending on allies or taking a solo lane with a jungler.

The item sequence might be variable depending on how well the enemies on your lane are doing or how the killer on the enemie team is doing.
Ex: Change sequence of "Thornmail" and "Force of Nature" if the main carry of their team is an AD Champion.
The first thing to buy would be the "Sunfire Cape" Because it's your main source of damage along with "Despair".
Once you have "Sunfire Cape" and "Thornmail", dont be afraid to go into battle solo versus 2 AD Champions.

Amumu is crucial to the sucess of your team!
First of all, Amumu's Ulti is the most strategic skill of him. Always try to use it to get a running away enemy or at a mid of a team fight, and always make sure to be supported by your team Assassines. As the amount of mana you use in a lane to farm up with "Despair" and "Tantrum", so use your Ulti then use "Clarity" not only to use do some Damage Over Time with your "Despair" and the "Sunfire Cape" tanking the other Team's skills and if your not in a Low damage dealer team your might even get out alive or be dead after your team takes down 2 enemies, giving your team an advantage.

Tips: Don't ever try to face "Vayne", "Irelia" or "jax" on your own and always focus your atacks on the enemy's team Tank.