Sutekh
The Oldest One was ostensibly a member of the Pantheon alongside the Toymaker and Maestro. (TV: The Devil's Chord [+]Loading...["The Devil's Chord (TV story)"]) His name followed the same scheme as that of the One Who Waits, another powerful being mentioned by the Toymaker and Maestro. (TV: The Giggle [+]Loading...["The Giggle (TV story)"], The Devil's Chord [+]Loading...["The Devil's Chord (TV story)"])
When Maestro discovered that the song Carol of the Bells, which had played on the night of Ruby Sunday's birth, was hidden "deep inside her soul" with enormous power, they seemed to recognise the power as that of the Oldest One. However, they added a frustrated "He can't have been there!", seeming perplexed as they tried to reason why he might've been. The discovery ultimately prompted Maestro to release Ruby, spitefully declaring that "this creature is very wrong" to a baffled Fifteenth Doctor. (TV: The Devil's Chord [+]Loading...["The Devil's Chord (TV story)"])