The Forsaken is a group of humanoid lifeforms that solo purpose is complete extermination of all human life except those of the forsaken. It is unknown where these radical humans came from but they seem to have a very unique understanding of Forerunner technology.
First encounter The forsaken were first discovered by the elite's in 2569 who stumbled across them wile doing a routine patrol of the Yanme'e's (also known as drones) solar system. The encounter was brief , the Elites attempted to communicate with the the single alien craft but it gave no response but instead destroyed all four of Elite vessels. The elite sent a response of thirty ships but could not locate the anomaly.
The Story It is unclear where these humans came from but there search for Forerunner technology has opened a wide variety of questions. they seem to know everything about forerunner technology including the location of several forerunner artifacts. the UNSC has came to this conclusion do to the fact that the only human planets they attack has some sort of forerunner connection. The first human planet to be glassed by the forsaken was Sigma Octanus IV shortly followed by Reach, and finally Earth. the loss of these three planets put a major dent in the UNSC but the remaining resources were put into the defense of there last shield planet, Jericho. there the UNSC met with the Covenant to form a new alliance between the UNSC, the Elites, and the covenant. the agreement was formed to stop the forsaken juggernaut from annihilating all life in the galaxy. since then the forsaken continued to systematically search and eliminate all life in the milky-way galaxy the only message received from them was this
"the son shall pay for the sins of the father".
in witch was only broadcast once on all frequencies throughout the galaxy and was not translated until they came to the realization that the message was a form of Forerunner dialect.
The UNSC was able to triangulate the position of the signal and sent stealth ships to recon the area. What they found was astounding thousands of ships around the new positioned shield world Onyx and a giant portal that was similar to the one leading to the arc from earth. the ships transmitted what they had found but were destroyed shortly after. The UNSC and the allied covenant forces annualized the data and determined that the portal was definitely Forerunner technology and that it must be a portal to a different Galaxy. With the new found information they devised a plan to strike with every ship they had and overwhelm the Forsaken forces guarding the portal. there hope was to locate a halo array in the Forsaken's home galaxy and activate it. the outcome however is uncertain do to the fact that the battle is still being waged because the next book is yet to be written.
Source this is based on a book about halo that is call "Halo:the Forsaken" this book is yet to be released but is to be published by 2010.