According to the latest novel in the HH series "Outcast Dead" the attack on Prospero happened AFTER the Dropsite Massacre incident. This lefts me wonder, why the Emperor chose to utilize his Exterminator Legions against Magnus (for whom his intentions were just to be apprehended and brought back to Terra to be judged for his crimes) and not against Horus, whose forces big E wanted destroyed.
This is not a post to provoke a flaming thread of the type "yeah, the authors have completely messed up the whole HH thing and its timeline, they do not have a clue what they write about, etc". I am just posting to hear for possible explanations to what seems a rather problematic piece.
What we know based on official fluff is:
1) Rogal Dorn WAS the praetorian of the Emperor and he was the one who was running all the operations while Big E was doing his thing with the Webway portal in the Imperial Dungeon. So, when the rebellion of Horus (because it's clear that before the Dropsite Massacre Incident, the acts of Horus had been underestimated) became know, Dorn was the one who coordinated every action to suppress it. Big E knew, but he did not get involved.
2) When Magnus contacts the Emperor and lays waste to Terra through his sorcery, Emperor is first hand witness and gets really pissed off. So, he sanctions immediately action against Magnus.
Based on the above I tend to think:
The Wolves were not loosed against Horus:
i) Because it was Dorn the one giving commands and it seems that the Wolves were taking orders regarding these matters only by the Emperor. Being the atomic weapons of the 30k Imperium, I do not think that big E would have shared the launch codes with anyone.
ii) As I said before, it seems that Dorn had underestimated Horus' treachery. After all, he send against him seven legions. Adding the Wolves would have been the definition of an super-over-kill.
iii) Simply all other legions were closer to Istvaan system than the Wolves.
They were unleashed against Magnus (although this seemed an overkill):
i) Because a) the other legions were occupied, b) Russ was the principal instigator behind the Nikaea motion, c) their nature as a legion made them suitable in resisting the psychic powers of the Thousand Sons.
ii) They happened to be closer to Prospero.
I would love to hear your thoughts.
P.S.1: I recommend without any second thoughts the book "Outcast Dead". Simply one of the best in the series, if you like good prose and lots, lots of pre-Imperium fluff. Just a tease: Thunder Warriors (yeap, the Proto-Astartes dudes) have a role (rather big) to play in this story .
P.S.2: It's a pity that only ADB stalks these forums. We could really use some info from Mr. McNeil.