To be able to get to the Sphinx, you will need to complete the quest "A Family's Legacy". This quest will unlock as part of the story in Sequence 7 and it'll task you with many different things. This is the same quest that you'll be doing as you're going for  Birthright.

After finishing this quest, you will be granted another quest named "Lore of the Sphinx". This quest will task you with heading up to Boeotia where the Sphinx's alter is. Once you arrive, the Sphinx will show up and will start asking you a set of questions. You're tasked with answering these questions correctly along with pressing buttons on the wall that match your answers. If you were to answer incorrectly, then the Sphinx will automatically kill you, causing you to force a reload.

Unfortunately, the questions that are asked are always at random. There are twelve different questions that can be asked, but only three will be asked. Be sure to remember your answers as you'll need to press in three different illustrated buttons that reflect what your answer is.

Once you've answered the riddle correctly, then you will get this trophy.