Here is a video that covers the full playthrough of the mission.
It will start after you speak to Gina once she has intercepted a message from Voss about the Sphinx excavation.
Head over to the Sphinx snd you need to sneak into the underground section to the left. Be careful, at this point you are easily recognised by most enemies, however there is a disguise in the Sphinx dig near the currently blocked wall.
To proceed, you will see the door requires a medallion, the disguise is just to the left.
Make sure you put it on on and you will need to head back out to find a gold medallion, which is close by in a tent behind the Sphinx
The disguise will get you so far but I was recognised after picking up the medallion and the Jack of clubs secret in the draw, however if you are quick enough you can dart back out and out run your pursuers back to the Sphinx.
Once the door is open and you move a little in past the pit on the ground, you will find a dark section, it's too dark to expose and there are no torches. You must now head to the village (left at the front on sphinx) to find a market trader who will sell you this item
Now we can finally proceed.
There will be a what looks like a glass door over hole in the floor, you can hook your whip above but can't swing through the door. You have to lower yourself into the pit a little before youe see the required route.
There will be a few mirror puzzles ahead that are fairly simple and some scorpions which can be dealt with via a torch but not too much to stop you proceeding in the tomb. Just refer to the video if you are stuck at any point.