You Do Need power plug adapter while traveling from Eritrea to Saint Helena.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Helena is G 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

Saint Helena

In Saint Helena, outlets of type G are in use.

Type G