The Lizard's Face (Main Quest)
This quest begins immediately after completing 'The Lizard's Mask.' Follow your marker to Aya, and speak with her to begin a horse race.
The race is winnable- just don't over think it. There aren't any shortcuts, or tricks, and you can't bump Aya into anything. Just go in a straight line and don't try anything funny.
Win or lose, you'll arrive at a cutscene that begins with Cleopatra failing to garner a warm reception in Memphis, and ends with Bayek going to slay the Lizard.
The trick here is that the Lizard is one of five identically dressed priests in the Temple. You don't have to sneak around or anything, but make sure the priest you kill is the one with a blue scarf, and a terrible cough. Use Senu to mark each priest's location.
For the priest, as with the race, don't over think it. You don't have to sneak around here, so there's no fancy maneuvering, and the Lizard won't be spooked if he sees you. Likewise, his scarf really is blue- not some confusing midnight blue or dark navy blue hybrid. Lastly, Memphis is a heavily polluted city, and many priests will clear their throats from time to time. The Lizard's cough is a multi-part thing with a gasp and wheeze, that causes him to hunch over. Listen for that, and you'll be sure to ice the right Anubis.
Another afterworld scene confirms the end of the Lizard, but not yet the end of the mission.
Follow your marker back to the palace for a final slew of cutscenes, and mission completion.
It's not really clear what the Lizard was trying to accomplish here, but perhaps the Snake's penchant for mass-murder and misery will be explained in due course.