NOVA bomb

First mentioned in Halo: First Strike, the NOVA is a special device that uses nine fusion 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..."

The resulting explosion detonated in-between the only known Covenant planet Joyous Exultation and its moon, Malhiem; scorching a quarter of the planet, shattering the moon, and the resulting winds on Joyous Exultation flattened cities and spawned tidal waves.

Trivia
The Nova Bomb was a bomb from the book Starship Troopers wherein it is described as "being abled to split a planet in half". It is likely that this inspired the Nova Bomb.