While I have enjoyed the tidbits that the Horus Heresy series has given about the legions and their primarchs, what I am enjoying most is the insights into Malcador the Sigilite.
Throughout the series, he appears in various places, and at first, he appears to merely be one of the many advisors around the Emperor, but as the series goes on, he is fleshed out into a multifaceted character.
In the Dark King/Lightning tower, he is shown merely as a means for Rogal Dorn to express his misgivings, and as a means for Dorn to come to a conclusion as to what they are.
In the first heretic, he is shown to have considerable power, both political and otherwise, during the censure of the Word Bearers. This passage established that he spoke with the full authority of the Emperor, something that the high lords of Terra did not enjoy until after the Emperor's interrment in the Golden Throne.
In Nemesis he is then shown to be the Emperor's master of assassins, in addition to his other duties. He is shown as being a master schemer, more than capable of competing with the primarchs in tests of wits and intelligence.
With the Garro books, he is seen assigning Garro to assemble a force of marines from many legions, some of whom are fellow loyalists from traitor legions for an as yet undisclosed mission. One of the telling parts was the weight of authority that the sigilite's mark provided Garro, one where he was able to assume command of a battle front controlled by another legion.
What this says for me is that Malcador was far more influential in the state of the 40k universe than the background of the Malcador tank would indicate, even taking into consideration of the precedent of the Leman russ and Macharius tanks being named after figures whose impact is well known.
While it is too soon to say with any certainty, it looks like malcador is responsible for at least two major factions of the imperium's might, the assassin temples and the Inquisition, and possibly more, depending on how he acted in the days following the end of the heresy.
This has made a character that I had previously seen as a supporting character that was little more than the reason why the malcador tank had its name into one of the most interesting characters in the 4ok mythos, even moreso than my three favourite Primarchs, leman Russ, Mortarion, and Sanguinius.