.. What's missing from the wall in the burial chamber?

It's hard to tell what shape it is, but I'll give you a clue and tell that you don't have it in your inventory when you open the chamber. Keep looking.

You'll find what you need in another room that you haven't found yet.

Finding the Second Tomb

What you need is in the little coffin in the cat tomb --

How do I open the little coffin?

-- and when you put that item into this hole in the wall, it reveals a whole new puzzle (sigh).

How do I solve the cobra puzzle?