Davros's accident

A Thal bombardment resulted in Davros being crippled, (AUDIO: Davros, Corruption) which the Sixth Doctor and the Eighth Doctor recalled as an "accident". (AUDIO: The Juggernauts, Terror Firma)

The accident
The event occurred one month after the death of Calcula, Davros's mother. Davros was in fact grievously wounded by a Thal bombardment of his laboratory in the Kaled Dome which cost him his taste buds, left arm and entire lower body and left his eyes with severe damage to the point where using them would cause great pain. (AUDIO: Davros, Corruption) Although the reinforced walls of the lab prevented the bombardment from killing the Kaled scientist, (AUDIO: From the Flames) he was forced to spend the rest of his life confined to a mobile life support system attached to a wheelchair, with an eyestalk-bulb in his forehead that gave him partial eyesight without the use of his own eyes.

After he was crippled, the Scientific Corps gave Davros a projectile poison injector to allow him to kill himself. He kept it but refused to use it. (AUDIO: Davros, Corruption) In his life-support system, Davros was identified as a mutant. (TV: Destiny of the Daleks) According to some accounts, he based the design of the Dalek casing off his life-support chair. (PROSE: The Whoniverse, et. al) However, the idea of what would one day be realised as the Dalek casing were already starting to form in Davros's imagination by the time of his accident. (AUDIO: From the Flames)

Legacy
By the time of his invasion of Earth, the now Emperor Dalek Davros created a clone body of himself which he intended to transfer his mind into. Observing the appearance of Davros before his "accident", the Eighth Doctor quipped that he was never really a "looker", whilst Davros himself admitted that he had forgotten what he once looked like. (AUDIO: Terror Firma)

Alternate timeline
In an aborted timeline, used the Anti-Genesis codes to travel from the Last Great Time War to Skaro during the Thousand Year War, and lured Davros into a trap; pretending to be Davros's uncle, the Master summoned the Kaled scientist into the observation room of the Kaled Dome, so that when the Dome was hit by a bomb, Davros died instantly. (AUDIO: From the Flames) This change in history allowed the Master to take control of the Kaled Scientific Elite in Davros's place and, by extension, create a race of Daleks obedient to him instead of Davros. (AUDIO: The Master's Dalek Plan) This timeline was eventually erased from existence by the Master's future self, a Master from a parallel universe, and the Dalek Time Strategist, restoring Davros's position as the true creator of the Daleks. (AUDIO: He Who Wins)