The Ancient one

A beggar grabbed Kaylin in the streets of Port Nyanzaru and shouted 'The ancient one beneath the Forbidden City gives birth to a terrible new god'. The snake men know! They know! He then seems to shake himself as if he were asleep and carrys on walking into the crowds.
The Guardians of the Gate believe the "Ancient One" could refer to Acererak  while the "Forbidden City" likely is reference to Omu.
Should the Guardians of the Gate encounter anymore Yuan-Ti they should look at interrogating them regarding Omu.
Following conversations with Saja N'baza, and what she revealed it is possible the Guardians of the Gate were wrong in assuming the "Ancient One" was Acererak and instead it could actually be Nsi .