This portrayal is fundemental to the development of the description of the elite group.
Without context, it is a bit difficult to say, but I would change 'the' for 'this'.
Taken alone, it needs some description of 'the elite group' such as 'of the elite group who secretly run the US'
Some portrayal:
This portayal is fundamental.
There is an elite group in Washington who secretly run the country.
This portrayal is of this (aforementioned) elite group.
This portrayal is fundamental to the development of the description of this (aforementioned) elite group.
or
This book is a portrayal.
This portrayal is fundamental to the development of the description of the elite group who secretly run the US.