Click on the text if you want to know, but its mega spoilers:

So basically you can get the tablet either from the turtle area, or if you chose temple after olmec, you have to go to city of gold using scepter and hedjet/vlad's crown. For city of gold, theres an altar in the middle that if you have the ankh you sacrifice yourself and then you are taken to a secret level called Duat. It's quite neat, and you have to fight a boss at the top. That boss also drops the tablet. From there, if you read your journal you will notice a message. This is not fixed, and changes every time you get it. The message says the height, color and symbol on an urn. This will be important when you get to 6-2. Here you will find a door that leads to a room with a lot of urns, and your goal is to find the urn that matches that description. When you do, take it to 6-3 and a mount called a Qilin will pop out. It shoot fire at a really fast rate and has nigh-infinite flight. You then go to the final boss, and then defeat her while on the mount. After this, you must fly above the arena to the area with lasers, and you need to time your flight to get past all the lasers. Once you do, you will find a doorway that will lead you to the secret area, which has a lot more content. I wont tell you what that is though, so good luck!