He disappears because you don't need help finding it anymore (Congratulations on doing so, btw! Many players have found the puzzle difficult.)
The Keeper's strength is a direct function of how many treasures you've raided from the pyramid. There are a few NPCs that mention this. If you've raided all of them, the Keeper will be at maximum strength, so your stats will need to be very high, or you'll need very good strategy and luck.