The Soul Bombs were extremely powerful explosive devices built by Urza during the Phyrexian Invasion, based on Thran design. They were taken by the Nine Titans to Phyrexia to destroy that plane. The only way of fueling them was by capturing a potent-enough soul. Barrin had argued that there was no ethical way to charge the devices, as he believed that no mortal crime could be severe enough to merit the immortal punishment of being used to fuel the bombs; and that the number of mortal souls necessary to power the devices was monsterous. However, Urza instead found a single immortal soul powerful enough to charge the devices. He intentionally included Tevesh Szat as one of the Nine Titans, predicting that Szat would in some way betray the group. Once Szat lived up to these expectations by murdering Daria and Kristina of the Woods, Urza felt justified in activating the kill rubric in Szat's Titan engine, fueling all the bombs with the planeswalker's life essence.
After Urza betrayed the Titans, he destroyed the master soul bomb, which was designed to set off the other bombs. However, the surviving Titans were able to activate the other bombs manually, devastating several of Phyrexia's spheres.