Board Thread:Warhammer 40k General Discussion/@comment-69.143.134.163-20150419161351/@comment-71.215.130.211-20180624004826

It's pretty much heavily implied through all the evidence on the two legions (and there is quite a lot in spite of them being erased from history) that one legion betrayed the Emperor and the other went mad. Hence the "Purged & Forgotten" moniker.

The "Purged" betrayed the Emperor, somehow, and were destroyed - likely by the Space Wolves. As the Lost Primarchs are said to be the cause of both Legion's fall, I'd say one of the Primarchs grew up in a civilization that contradicted the Emperor and His mission, was forcefully removed from it and forced to watch his home destroyed by the Emperor himself, and grew to resent the Emperor for this to the point he turned his own Legion against him. He may have been raised by xenos, was raised to believe in peace with xenos and had many xeno friends, or even came from a very religious world. Whose to say, really?

The "Forgotten" was lost, having gone mad due to genetic issues like that present in the Blood Angels most likely - although it was probably worse than what the Blood Angels go through - except it was just the Primarch, not the entire Legion, and his madness resulted the destruction of his Legion. Either by the hands of the Space Wolves, or the hands of something else. Alternatively, they could have been infected by some kind of virus their Primarch spread deliberately amongst his men falsely believing it would benefit them or may have very well been the first convert to Chaos among the Space Marines, his taint by Chaos being stronger than the others who later rebelled, and led his Legion astray unwittingly.

Regardless, it's mentioned that the Primarchs failed - not the Legions themselves - and therefor the Primarchs must have led their Legions astray through some manner which warranted in at least one of, if not both of, their destrucion by the Space Wolves. If only one was sicked by the Wolves, the other was likely destroyed through it's Primarchs actions.

Ultramarines absorbing a Legion is just a rumor, it's more likely they took over the former worlds of one or both of the Legions - hence their size increase around the same time.

Blood Ravens are Loyalist remnants from the Thousand Sons, this is pretty much all but confirmed at this point as the hints to it are heavy and plentiful.

Female Primarchs is just... stupid. I'm sorry, but just because the Emperor was against having female Primarchs doesn't mean he would've outright erased two Legions just because their Primarchs were female. That's an incredibly irrational overreaction.from a guy who militantly champions rationality. Besides, it's outrighted mentioned there were 20 brothers/sons by both the Emperor and the other Primarchs, so no, there were never any female Primarchs. Female Primarchs wouldn't even make sense seeing as the Primarchs, like all Space Marines, genetically come from the Emperor himself and have no mother. This is precisely why no female Space Marines - Primarchs or otherwise - don't exist. Yes, Malcador mentions the possibility of female Primarchs, but this likely would have been a much more difficult thing to do as we don't even know if the Emperor himself has a mother. His very being is steeped in so much mystery we virtually know nothing about the man except his beliefs and very recent history. Everything else is speculation. For all we know he was born from lava at the formation of the Terra through psychic means or is the science experiment of some xeno race long dead. If he doesn't have a mother himself, it would be difficult to make female offspring from him. Likewise, as the Primarchs and Space Marines don't have mothers they logically can't be female either. Theoretically they might not even be able to have daughters, though admittedly I'm just speculating there. You need a male to make a male and a female to make a female. There was a recent development in science a few months ago I believe that sorta proved this, it gave female same-sex couples the chance to procreate with one another without the need for a surrogate father - the only drawback was they could only have daughters as a result of the lack of male genetic material. That's the extent of that technology in the current age, and as the Imperium somes to be fairly primitive for a interstellar empire I doubt they have the technology to give a man a daughter without female genetic material.

Ultimately the Primarchs failed their Legions and the Emperor, and the Wolves were unleashed for the first time as a result. One destroyed itself, as well. Alternatively, one of the Legions destroyed the other for some unknown reason and the attacker was labelled the "Purged" while the defender was the "Forgotten", and they were both erased to cover up the entire incident entirely. Space Wolves probably destroyed the attacker before the cover-up, but it's pretty much unofficially confirmed the Space Wolves destroyed one of the Legions.