This was directly acknowledged by Alpharius in his Primarchs series novel, where he says he he suspects at least one of the lost Primarchs met his end at Leman Russ and the Space Wolves' hands (although he also admits that he has no evidence to back this up). It would explain their sobriquet 'The Emperor's Executioners', but it seems that they nominated themselves. It has been repeatedly hinted that the Space Wolves were involved in wiping out their Legions.Riddle for the Ages: Don't expect to be actually knowing for sure about these guys anytime soon.Historical In-Joke: They were originally a reference to Rome's 17th, 18th, and 19th legions, who's numbers and symbols weren't used again after being wiped out.

Fast-forward to its release, and the only mention of the two Legions and their Primarchs in the drama is an off-hand comment about how, out of the twenty Space Marine legion symbols on the door to the Emperor's old gene-labs, two had apparently been scratched off or worn away. When the cover art was revealed prior to release, many fans speculated that it would finally reveal key information about the Legions and/or their fates (something which Games Workshop even hinted at in the article revealing the cover art).

Covers Always Lie: The cover art for the audio drama The Sigillite depicts Malcador the Sigillite sitting on a throne, which has a pair of skulls - each marked with the Roman numeral for one of the lost Legions - integrated into each arm rest.

Ambiguous Ending: At some point, they simply disappeared from all the records, and the only people who know what happened are quick to cut off any train of thought that would lead to answers.Primarchs II and XITwo Primarchs who were, along with their Legions, completely erased from all records and history so thoroughly, nobody knows what happened.

Horus heresy timeline