Henry Arbinger was the alias adopted on Earth by Maestro's Harbinger. A being created by Maestro, he referred to them as his "daddy". He resembled an unremarkable-looking human child, and took piano lessons with Timothy Drake, the composer who had rediscovered the Lost Chord capable of summoning Maestro into the universe, until Maestro decided to reveal themself. After they did so, Harbinger seemed to pop out of existence, leaving only a pile of clothes.
However, the child later reappeared, looking rather vexed, after Maestro was banished, looking on as the Fifteenth Doctor and Ruby Sunday sang and danced to "There's Always a Twist at the End". (TV: The Devil's Chord [+]Loading...["The Devil's Chord (TV story)"])