A child of the Toymaker, the Maestro is the living "essence of music", feeding off harmony and tune to become all-powerful. Wishing to covet, absorb and claim all songs in the universe, they sought to quell the music of the spheres and become the only living being left in existence.
After the Doctor cast salt at the edge of the universe, where the walls between realities were at their thinnest, the Toymaker and his legions were able to pass through into this universe. While some of these forces chose to remain silent, others made their presence known, sewing chaos and destruction.
Among these forces was the Maestro. While unable to fully manifest themselves in reality, they were able to send their son Harbinger to Earth. There he would search for a musical progeny clever enough to play the long-lost "Devil's Chord" and fully summon their father. This was achieved in 1925 when he manipulated the composer Timothy Drake to play the tritone.
Turning on their summoner, Maestro sucked out all the music within Drake's heart, before continuing with the rest of humanity. Their reach stretched through the whole of history, drastically altering the timeline till at last by 2024, all that was left of London was the sounds of a nuclear winter and ruin.
First discovering their influence in London, 1963, the Doctor and Ruby caught their attention after the musician played a piano on the roof of EMI Recording Studios. Recognising their laugh as the Toymaster's Giggle arpeggio, the Doctor turned and ran, terrified of the threat they posed as a member of The Pantheon.
Giving chase with their cosmic tuning fork, Maestro was momentarily confused by the Doctor's Sonic screwdriver, which he used to create a wave of silence that blanketed the surrounding area. Amused by this creativity, they disrupted the silence with their fork and a puddle. However, before they can track them down, they are distracted by a distant piano player, which allows the Doctor and Ruby to escape. The piano player was not so lucky.
Following the Doctor and Ruby to the ruins of music-less London 2024, Maestro threatened the pair, revealing themself to be the Toymaker's child. Lamenting about how their "daddy" treated them over the years, they thanked the Doctor for banishing the Toymaker, since now they had no competition in their plans to bring silence to the universe. Attacking the TARDIS with musical notes, the Doctor and Ruby barely escaped with their lives, returning to 1963.
Capturing Ruby at the studio, Maestro was baffled to discover the Carol of the Bells was buried within her heart, enhanced by a power they recognised as that of "the Oldest One". Thrown off their game, Maestro was challenged to a music battle by the two time-travellers, ending with the Doctor almost figuring out the Lost Chord needed to banish them. Although he got the majority of the notes right, the last one was not the correct in the sequence, leaving him vulnerable to the Maestro's second wind.
Arrogantly toying with their victims, trapping the Doctor in a drum and Ruby in a double bass, John Lennon and Paul McCartney happened to wander past the piano, and completed the sequence based on their own musical "genius."
Dragged kicking and screaming back into the piano from which they'd first appeared, they were banished from the universe. However, before they went, Maestro snarled at the Doctor that the coming of the One Who Waits was almost upon them.