Tomb's can be found all across Egypt. The first tomb that can be seen in the game is located in Siwa called the 'Mountain of the Dead Tomb', which is located North West of the Siwa's main fast travel location as seen in the image below. This tomb is also connected to the side quest 'Hideaway - Level 3'.
This quest will require Bayek to save a young boy named Chenzira who has stumbled into the Tomb as he was being chased by a pack of Hyenas. Bayek will need to find a path to save the boy. Once you have found the boy and reached the end of the tomb, interact with the Ancient Tablet to receive the achievement.