Nova bomb

First mentioned in Halo: First Strike, the NOVA is a special device that uses ten nuclear warheads in a ring formation. Once detonation occurs the lithium-triteride fusion cores combine together and the resulting blast is multiplied hundred-fold. Nick-named "planet-killers", these have enough explosive power to destroy an entire planet.

In the novel, they were left on Reach and "repurposed" by Admiral Whitcomb. His reasoning was that "either the Covenant pack those bombs up and take them home for study, a possibility that I hope to God happens. A bomb like that could crack their homeworld in half.  Or, the bombs stay here, and they'll stop the Covenant on Reach..."