Board Thread:Warhammer 40k General Discussion/@comment-69.143.134.163-20150419161351/@comment-5832352-20170101045527

You could just as easily state that the 2 Legions were the spiritual predecessors to the Grey Knights.

The Emps had foresight. He may have created 2 Legions not for his war of unification, but to fight Chaos itself wherever it appeared.

However, early on in the Crusade, he did not encounter it, so the Legions were redirected towards conventional warfare.

Later, the Emperor started to run into the taint of Chaos among the stars, he needed his 2 anti-Chaos Legions for their true purposes again. So both legions return to their original purpose.

But, the Emps forced his 2 sons to be erased from history so that they could focus on their fight. In time, the wars they fought are forgotten and purged, much like the Inquisition does on any matter of Chaos.

However, by the time of the Heresy, the Legions were spent. Even the 2 Primarchs dead. (in one case, killed by the Space Wolves. They were corrupted by what they were fighting against.  The space Wolves were brought in under the guise of dealing with Traitors or whatever.)

The other Legion, all that was left were their gene seeds. The "normal" was implanted into aspirants, and the aspirants handed over to the Ultramarines. The geneseed that was the purest, from the greatest champions and psykers (Etc etc) was reserved for any future iteration that would be needed to fight the forces of Chaos.

The need came sooner than expected. The Grey Knights are born.