You Do Need power plug adapter while traveling from Sierra Leone to Nepal.

Here is Why You Need An Adapter

The outlet and plug type used in Nepal are C, D and M and in Sierra Leone are D and G. This is how they look like.

Sierra Leone

In Sierra Leone, outlets of type D and G are in use.

Type D

Type G

Nepal

In Nepal, outlets of type C, D and M are in use.

Type C

Type D

Type M