A laser screwdriver was a laser device used by the Master, unlike the Doctor's sonic screwdriver. They were larger, bulkier and generally used as a lethal laser weapon instead of a multi-functioning tool. (TV: The Sound of Drums, Last of the Time Lords)
As a fatal weapon, a laser screwdriver shot highly lethal laser energy, in which the force produced was enough to send a human flying several feet. However, its effectiveness dwindled when used against the Doctor, after he was protected by a psychic field generated by the Archangel Network.
Apart from the killing function, a laser screwdriver was capable of artificially ageing a target. The Saxon Master used the function to age the Doctor using a sample of his genetic code taken from his severed hand, turning him into an aged creature. (TV: The Sound of Drums / Last of the Time Lords)
A laser screwdriver that the Master eventually used whilst onboard a Mondasian colony ship, (TV: The Doctor Falls) following the Doctor's acquisition of his previous screwdriver, (GAME: TARDIS) was capable of killing Mondasian Cybermen (TV: The Doctor Falls) and Neomorph Cyber-Lieutenants, (PROSE: Alit in Underland) as well as radically evolved "weapons grade" Cybermen when used in conjunction with the Twelfth Doctor's sonic screwdriver, Missy's sonic umbrella, and a Mondasian Cyberman's cyber-gun. (TV: The Doctor Falls) A lower setting used to burn small incisions could disable a patient. This laser screwdriver could also burn through deadlock seals, generate an electrical pulse powerful enough to kill CyberNeomorphs and their Cyber-Leaders when fired at a damp metal surface, function as a torch, (PROSE: Alit in Underland) and call the inertia lifts of the colony ship. At its maximum setting, this laser screwdriver was capable of severely injuring a Time Lord beyond the point of regeneration. (TV: The Doctor Falls)
He first unveiled the device to the Doctor aboard the Valiant, when he used it to stop Jack Harkness from intervening with the Toclafane invasion and age the Tenth Doctor. He compared it to the Doctor's sonic screwdriver, stating, "Laser screwdriver. Who'd have sonic?" (TV: The Sound of Drums) He later used the screwdriver to shoot and kill Thomas Milligan. He later attempted to kill the Doctor with it but was unable to penetrate the psychic field he had formed, the weapon sooner being forced from his hand when he turned it on Martha and her family. (TV: Last of the Time Lords)
The device was later seen in the Doctor's TARDIS. Amy Pond attached it to the console to power the ship's tractor beam and pull the Eleventh Doctor out of a riptide in spacetime. (GAME: TARDIS) It remained in the console after that. (GAME: The Gunpowder Plot)
The Master later acquired a modified version which he used on the Mondasian colony ship. (TV: The Doctor Falls) He attempted to harm the Twelfth Doctor with it but it was wrestled from his grip in their struggle. He later used it to destroy numerous Mondasian Cybermen as they tried to capture him for conversion. He used it again in combination with Bill's cybernetic blaster to disable an improved model during the battle. After Missy stabbed him in the back and left him to regenerate in the elevator, the Master (refusing to allow his future self to help the Doctor) blasted her in the back with his laser screwdriver. The Master told Missy that she could not regenerate as he gave her the full strike on maximum power, leaving her to die as he departed. (TV: The Doctor Falls)
Evidently the Master abandoned the weapon upon regenerating into Missy, replacing it with her multi-purpose device capable of vaporising targets and interfacing with technology (TV: Dark Water) and her sonic umbrella. (TV: The Doctor Falls)
Behind the scenes
Conceptual art released on the BBC website as well as conversations with the cast and crew reveal that the laser screwdriver was intended to be larger and more intimidating than the Doctor's sonic screwdriver, meant to be "shown off". This, as well as its lethal and dangerous settings, fit in line with the Master's portrayal as an antithesis of the Doctor. It was also meant to look like something the Master constructed on Earth, out of local materials.
A toy replica of the device was released, similar to the Doctor's sonic screwdriver. It does not feature a pen or any removable parts, save for the battery compartment. A single press of the trigger extends the spring-loaded tip and activates the laser sound effect, along with three bright yellow LEDs. Holding the trigger for about one second activates the ageing sound effect. The triple-ended tip does not extend further, as it does on the show. While slightly smaller than the on-screen prop, it does feature the crack-textured setting adjustment dial, with an integrated ratchet joint for further sound effects. A later revision to the design left out the ratchet in favour of a more accurate appearance.
Fans of the original series noted that the laser screwdriver was a departure from the Master's other favoured weapon, the Tissue Compression Eliminator. When questioned about this, the current series' cast and crew commented that the Master may have simply preferred to use something different for the element of surprise.
The non-narrative source Doctor Who: The Visual Dictionary gave further information about the laser screwdriver. The laser screwdriver incorporated laser technology, which shot out lethal laser beams at an enemy from its tip.
The War Master's screwdriver, as seen on the cover of Rage of the Time Lords, is visibly made of at least parts of Character Options's sonic screwdrivers building sets, including two from the "Trans-Temporal Sonic Screwdriver". The head is also very similar to a fanmade screwdriver.