After finding the gates to Atlantis, the new questline unlocks and starts with the Sphinx.
She asks three random questions, I'll give you the answers to all of them. Then choose the matching symbols of the answers in order. E.g. if the answers are Moon, Sun, Star, you find the glowing symbols matching those to end the quest.
What can bring back the dead, make you cry, make you laugh, make you young; is born in an instant, yet lasts a lifetime? Memories
This thing all things devour: Birds, Beasts, Trees, Flowers; gnaws iron, bites steel; grinds hard stones to meal; slays kings, ruins towns;
and beats high mountains down? Time
Some try to hide, some trt to cheat; bit time will show, we will always meet. Try as you might to guess my name; I promise you'll know, when you I do claim Death
As small as your thumb, I am light in the air. You may hear me before you see me, but trust I am there. Hummingbird
I'm Alive, but without breath; I'm as cold in life as I am in death,
I'm never thirsty, though I always drink Fish
I create my lair with earthen string, and dispatch my prey with a biting sting Spider
What is always old and sometimes new; never sad, sometimes blue; never empty, but sometimes full; never pushes, always pulls? The Moon
Never resting, never still; moving silently from hill to hill; it foes not walk, run or trot; all is cool, where it is not The Sun
At night they come without being fetched, and by day they are lost without being stolen Stars
In spring I am gay in handsome array; in summer more clothing I wear; when colder it grows, I fling off my clothes; and in winter quite naked appear Tree
What is large, yet never grows; has roots that cannot be seen; and is taller than all the trees? Mountain
What can run, but never walks; has a mouth, but never talks; has a head, but never weeps; has a bed, but never sleeps? Rivers
Once you've done answering her riddles, return to her and she'll tell you to hit the symbols of your answers in order. Once you've done that, the quest ends and the Sphinx is defeated.