Quote Originally Posted by Fangschrecken View Post
To resurrect a semi-dead thread, after reading the Forgeworld Heresy books it's heavily hinted (basically spelled out) that at least one of the Legions was destroyed in the Rangdan Xenocide. Said to be a truly apocalyptic war, which is probably the most we will ever get out of it.
Both Lorgar and Sanguinius have expressed fear about ending up like one of the Lost Legions, and the SW seem to take some pride in having already destroyed one loyalist legion well before the Heresy.

So there we have it. One destroyed in a huge conflict, one was purged for somehow displeasing the Emperor.

Some surviving members may or may not have been folded into the UM, though the author who came up with that has outright stated that it was just meant as a false in-world rumor.