The southern region of Egypt was known as

Lower Egypt

Explanation: There was Upper and Lower Egypt, but the southern region is Lower Egypt.

Upper Nubia

Explanation:

The southern portion, which extended north to the southern end of the second cataract of the Nile was known as Upper Nubia; this was called Kush (Cush) under the 18th-dynasty pharaohs of ancient Egypt and was called Ethiopia by the ancient Greeks. United Egypt politically when he gained control of both Upper and Lower Egypt. Somewhat confusingly, when you look at a map of this area, Lower Egypt is the delta region in the north, and Upper Egypt refers to the southern portion of the country, which is upriver from the delta.