This is an expansion for the base game 5150.
From the publisher's website:
This book will introduce you to eight new aliens. Inside ILLEGAL ALIENS you will find complete descriptions for each of these alien races. These include pictures and the following information:
Who these aliens are.
Their current history.
The type of Encounters they have.
Their Prime Motivator.
Their small unit organization.
The Attributes they possess including new ones not previously mentioned in 5150.
The weapons they use at squad level.
Any available support weapons.
What reinforcements are available.
What type of vehicles they can utilize.
And finally complete stats for their weapons.
In addition we have included one scenario that introduces yet another new alien for a total of nine! This scenario features a new alien known simply as the "Night Terror".
So without further adieu, let us introduce you to your new neighbors!