Assassin's Creed Origins

The Arsinoe sundial obelisk referred to in the description is in the northern region, Arsinoe, and in the northwest of this area. The screenshot below shows exactly where on the map it is.

Once there, you have to climb up to the top and perform a leap of faith between 10 am and noon. If you use the Dawn and Dusk skill to fast forward time, it'll start you around 6 am at dawn. You'll then have to wait around 10 minutes or so for the game to reach 10 am. The time is indicated by the shadow of the sundial on the lines on the ground. The line sticking straight toward the gateway is noon, and two lines to the left is 10 am. The screenshot below shows the shadow of the sundial at 10 am. Wait until the shadow moves a little farther past the 10 am mark and then jump off toward the noon line to unlock the achievement.