Reincarnism

Reincarnism is a spiritual concept and minor human religion that had originated on Minorca, sometime in the 2900s. The belief was evidently heavily influenced by the concept of reincarnation, or transmigration of the soul, which was also present in numerous ancient human religions originating on Earth.
Reincarnism varies from the ancient views of reincarnation, in the sense that it places emphasis on "memories" of past-lives suggesting the existence of two long-departed, advanced, sapient races whom had been feuding for hundreds of years. The specifics of this plot are never identified, and "memories" without relevance to this plot are often dismissed and aren't widely recognized by the reincarnist community.
Furthermore, it is believed that those who claim to recall such retrospections each contain the "souls" of individuals who had been spared death in the massive holocaust, by means of salvation, or the slow, steady process of soul integration. A point is made the souls of those whom had chosen salvation were to be integrated into the bodies of modern humans millions of years after their departure. Because these souls supposedly, come second to our natural souls and mind, they are incapable of controlling ones body, only sharing in as well as living out the remainder of the bearer's life. Through hypnosis and such processes, it is believed that certain memories of the seconds soul may be vividly relived. Once one medium is deceased, the secondary soul may pass to another carrier. 
The two feuding species are rarely distinguished, and that whom's souls had transgressed into humans is never clearly identified from the two. The cause of the conflict is unknown, however, it is believed to have lasted thousands of years.
==Research==
Strangely enough, many other races share similar concepts in their ancient, and sometimes even modern cultures. While reincarnism is only regarded as a religion amongst humans, such cases have been taken into account and are often considered in argument.
The memories mentioned in reincarnism have been related to a recently discovered topic regarding means of memory implantation, specifically . Researchers who have scrutinized most claims, however, don't agree with this due to numerous inconsistencies. While the possibility of these visions being of Forerunner-origin is yet unclear, it has recently been proposed that their point of origin may in fact be a far more ancient Geas.