How much energy is there in 1 mol of diamond and how can you get it?
Diamond is just one of the allotropes of carbon. There is nothing unique about diamond that allows it to store significantly more energy than any other compound. It is true, however, that much energy is required to separate carbon atoms in a diamond lattice. This is due to the high bond dissociation energy of a carbon-carbon covalent bond.

Although bond dissociation energy in a C—C bond is relatively high, it is dwarfed by the energy predicted to be released by the mass-energy equivalence theorem.

$E = mc^2$

The nuclear mass of 1 mole diamond is about 12 grams. So the theoretical energy released in a fusion reaction involving one mole of diamond is around 1*$10^9$GJ. 

I don't think this is the answer you were expecting. But this is the amount of energy in 1 mole of carbon. The method of releasing all this energy is a different story. Nuclear fusion would release a significant portion of this energy but not all of it.