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

Here is Why You Need An Adapter

The outlet and plug type used in Swaziland is M 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

Swaziland

In Swaziland, outlets of type M are in use.

Type M