NOVA bomb

"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 yield a hundredfold. 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..."

- Danforth Whitcomb's recording stored in the NOVA Bomb that destroys Malhiem

The NOVA Bomb is a powerful thermonuclear bomb of the United Nations Space Command with the power to destroy an entire planet.

Theory
The NOVA Bomb itself is nine fusion warheads encased in lithium triteride. When the warheads are activated, the lithium triteride shell focuses the blast upon itself, multiplying the destructive power a hundredfold, giving the NOVA Bomb an unprecedented payload for its size. Its capability to destroy a planet gave it the nickname of "planet-killer". Even if the power of this device has never been estimated, 9 warheads of 30 megatons each multiplied by 100 (coeificiant) = 22gigatons.

History
It was intended that the NOVA Bomb be used in space against the Covenant fleet during the Battle of Reach. 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 two. Or, the bombs stay here, and they'll stop the Covenant on Reach..."

The resulting explosion detonated between a Covenant outpost planet Joyous Exultation and its moon, Malhiem, scorching a quarter of the planet and shattering the moon, and the resulting winds on Joyous Exultation flattened cities and spawned tidal waves. This was actually extremely unfortunate as the Elites, lead by Imperial Admiral Xytan 'Jar Wattinree could have shattered the Brute fleet, thus effectively destroying the Covenant. This could have turned the tide of the war for both the UNSC and the Covenant Separatists. It severely damaged 3 quarters of the fleets that were there, which was over 300 ships.

Delta Halo
In Ghosts of Onyx, Cortana requests that the UNSC launch a NOVA Bomb around Delta Halo and High Charity to stop the spread of the Flood and destroy the Covenant Holy Capital. However, this request was not granted, because High Charity survived and landed on the Ark. It could also be that this request was granted, but that High Charity just got out on time.

Trivia

 * The Nova Bomb was a bomb from the book Starship Troopers, wherein it is described as "being able to split a planet in half." It is likely that this inspired the Nova Bomb.
 * A Nova Bomb is also the common name for the heavy bomb used in Starfox games. Though not nearly as powerful as the Halo and Starship Trooper bombs of the same name, the Nova Bomb is one of the most powerful weapons you can use in the Starfox series.
 * The Nova Bomb also appears in the Andromeda TV series. In that show it is a weapon capable of causing stars to go into the Supernova stage, therefore destroying the system around it.