S-P-O-I-L-E-R-S Ahead, trespassers will potentially be very irritated.
*** Spoiler - click to reveal *** After you figure out what has been poisoning the holy bull in the temple and rescue the twin priestesses brother, you will be tasked with killing the Lizard. The trick with this assassination though, is that he is wearing an anubis mask (as he is a priest to that deity)
along with five other priests. You have to find the one with a blue scarf and a persistent cough and be rid of him!
Now I got this right first time so if anybody would like to comment and tell me if the game desynchs when you get the wrong one I'd greatly appreciate it!
As always you will have a conversation with him in the realm between, then upon your return to the game the shiny achievement shall be yours.
Let's keep cheevin' together!