C-12 shaped charge

The UNSC C-12 Shaped-charges or C-12 SCs are used mainly for heavy demolitions and can also be used as weapons. This compound is extremely dangerous and very volatile. A small 4x4x4 inch cube alone can level a five story building in seconds. It's solid, but malleable enough where it can be shaped or bent into different forms to stick to surfaces better and fit into tight spots and is detonated with electric-shock detonator sticks or remote detonators.

C-12 is also hefted in medium sized backpacks called "Damage Packs" or "Blow Packs". Two Damage Packs contain enough C-12 to blow through three meters of UNSC standard armor plating.

Trivia

 * During World War II there was a reasonably powerful explosive called C-2, which is a precursor to C-3, modern day C-4, and possibly to C-12.
 * C-12 is also related to C-7.
 * C-4 is short for "Composition 4", therefore it is likely that C-12 means "Composition 12." It is also likely that it is simply a codename based the order in which it was developed, not an indicator of its explosive power (i.e. being 3 times more powerful than C-4. In real life, C-2 has more explosive power than C-4, but is also more unstable, and thus newer compositions were designed for stability and to be moldable, not to have extreme power for which other explosives already exist).
 * If enough C-12 is provided, it is able to penetrate Titanium-A armor, the armor that is commonly mounted on UNSC warships.