Mana bombs are powerful explosive devices developed by Kael'thas Sunstrider's blood elves for use against enemy spellcasters. The exact workings are unclear, but it appears to utilize some kind of massive mana burn that kills its intended targets almost instantly.
In Burning Crusade
This section concerns content exclusive to The Burning Crusade.
Kael'thas utilized these vile weapons to remove what he perceived as "obstacles". He used this weapon twice: a Broken draenei from Tuurem suicide-bombing the Cenarion Thicket in Terokkar Forest, and bombing Kirin'Var Village in Netherstorm. A third was prepared in the courtyard of Firewing Point, the closest of Kael'thas' outposts to Shattrath City, for use against either Allerian Stronghold or Stonebreaker Hold (the Alliance and Horde bases, respectively, near the edge of the Bone Wastes) or possibly Shattrath itself.
After a series of quests investigating what happened to Cenarion Thicket and discovering the blood elves' complicity, adventurers from both sides were tasked with attacking Firewing Hold and setting off the bomb before it could be used; the final code to set off the bomb was held by a Blood Knight named Sharth Voldoun, who resided at the top of the tower high above the courtyard.
In Jaina Proudmoore: Tides of War
The city was destroyed by a mana bomb, created by the blood elves using the Focusing Iris, by dropping it from a goblin zepplin. The blast killed most of the Alliance defenders, including , the 7th Legion, and many of its commanders. Jaina's gnome apprentice, , was also killed in the blast.
Jaina's eyes and hair were mostly turned white due to the blast and the violence brought on by it. The only known survivors were , , Jaina, and Kalecgos of the Blue Dragonflight. The ruins of Theramore were described in the novel as having a purple-pink hue around it, having been caused by the mana bomb. This particular mana bomb was described as the biggest in Azeroth's history. This event caused the war between the Alliance and Horde in the World of Warcraft: Mists of Pandaria expansion.
In Mists of Pandaria
This section concerns content exclusive to Mists of Pandaria.
As the apparent mechanics of Mana Bombs and the known mechanics of Arcane Bombs (which are crafted by Engineers) are somewhat similar, it could mean Mana Bombs are an extremely powerful variant of Arcane Bombs.