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..."

In Halo: Ghosts of Onyx it is discovered that the Covenant did in fact take the NOVA device with them. It was being transported aboard the Fleet Carrier Sublime Transcendence when the Grunt, Unggoy Kwassass, activated the device. It gave the message: " This is the prototype Nova bomb, nine fusion warheads encased in lithium triteride armor. When detonated it compresses its fusionable material to neutron-star density, boosting the thermonuclear yeild a hundrededfold. I am Vice Admiral Danforth Whitcomb, temporarily in command of the UNSC military base Reach. To the Covenant uglies that might be listening, you have a few seconds to pray to your damned heathen gods. You all have a nice day in hell. " (Page 244, Nylund)

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.