Logos Historica Verita
The Logos Historica Verita (Literally Discourse of Historical Truth) is a new Imperial organisation formed by Primarch Robo Jellyman after nearly suffering a brain aneurysm from trying to read the absolute dumpster fire that is, the Imperium's historical records. Members of the Logos Historica Verita are called Historitors.
Overview
In a nutshell, it is Guilliman's way of trying to unfuck the ongoing contradictory clusterfuck of Imperial record keeping.
On a more detailed explanation however, it is an organisation made for historians, by historians and of historians. After Big Bobby G awoke during the Gathering Storm, he quickly realised that the Imperium's 10,000 year history was catalogued in an extremely unreliable, secretive, and often contradictory fashion. Guilliman closing up the Library of Ptolemy on Macragge has less to do with him being paranoid over Imperium Secundus and more to do with trying to pull the breaks to figure out what the hell is going on in the history department, contrary to popular belief.
Due to the fact that 10,000 years worth of history will eventually have entire millennia's worth of context be unceremoniously lost by Administratum red tape, [REDACTED] by Inquisitorial meddling or twisted by Ecclesiarchal decree, trying to figure out the accuracy of the Imperium's history is one giant lasagna of a shitshow. No surprise than, that the Logos was created as a means to rectify this problem and create the first true reliable history of the Imperium, which would then be incorporated into Guilliman's new and shiny Codex Imperialis TM book.
The Logos is made up of hundreds of skilled scholars and historians, hand-picked by Guilliman, and chosen for their more liberal views of historiography. The Logos' objectives and policies stand in stark contrast to the Ordo Chronos, who aren't big fans of them and has turned to violence in response to Guilliman's reformations given that, you know, the Inquisition's habit of censoring shit is not gonna go well with the organisation that is tasked with exploring sensitive topics for the wider public audience. Granted, the only thing the Ordo Chronos can do is grumble and make indirect threats, because woe betide the dumbass who has to explain to a Primarch on why they thought it was an excellent idea to murder one of HIS hand-picked historians. Other parts of the Inquisition also aren't too chill about the Logos given that they would eventually pry into sensitive Inquisitorial intel, whilst the Ecclesiarchy is quite miffed that the way the Historitors would portray aspects of the Imperial Cult would be less flattering as they originally make out to be. But again, nothing else they can really do unless they like to entertain the idea of entering a staring contest with the Big Blue himself.
As of M42, the Logos is estimated to operate around 500 men and women and a thousand support staff. So it is a pretty small but crucially important organisation.
Tl;dr, it is the 42nd Millenium's more modern and useful reincarnation of the old Remembrancer Order.