So speak to the big sphinx there and it tells you that it wants you to kill the manticore for it. Trying to remember what sub-area the manticore is in but it will appear on your mini-map anyway. When you fight the manticore, be quick as it has a mate that will join in the fight.

The dungeon for killing the Manticore is usually in the bottom left or bottom right of the map. The door you are trying to open is for the final dungeon of the Ossean Wastes. You can finish it earlier, of course, if you like, but it's intended to be the hardest dungeon of the map.

In Stygian Wastes lurks beast of yore.
Part man, part drake. Manticore!
Lay it's pelt before my eyes
and passage through shall be your prize.