You Do Need power plug adapter while traveling from Eritrea to South Sudan.

Here is Why You Need An Adapter

The outlet and plug type used in South Sudan are C and D and in Eritrea are C and L. This is how they look like.

Eritrea

In Eritrea, outlets of type C and L are in use.

Type C

Type L

South Sudan

In South Sudan, outlets of type C and D are in use.

Type C

Type D