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 CrusadeThis section concerns content exclusive to The Burning Crusade.
Kael'thas utilized these vile weapons to remove what he perceives as "obstacles". He has 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 is prepared in the courtyard of Firewing Point, the closest of Kael'thas' outposts to Shattrath City, for use against either the 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 are tasked to attack Firewing Hold and set off the bomb before it can be used; the final code to set off the bomb is held by a Blood Knight named Sharth Voldoun, who resides at the top of the tower high above the courtyard.
In Jaina Proudmoore: Tides of WarThis section concerns content exclusive to the Warcraft novels or short stories.
It is revealed in the novel Tides of War, that the city is destroyed by a manabomb created by the blood elves using the Focusing Iris and dropped from a goblin zepplin. The blast kills most of the Alliance defenders, including , the 7th Legion, and many of it's commanders. Jaina's gnome apprentice, , is 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 are , , Jaina Proudmoore, and of the Blue Dragonflight. The ruins of Theramore are described in the novel to have a purple-pink hue around it, having been caused by the manabomb. This particular manabomb is described as the biggest in Azeroth's history. This event caused the upcoming war between the Alliance and Horde, in the expansion "World of Warcraft: 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.