@mystic_taboo 'GeeDubs' is a (not entirely) affectionate knickname for Games Workshop (often abbreviated to GW by wargamers, which is the source of the GeeDubs term), the tabletop wargaming company that owns the Warhammer 40,000 IP, created the original tabletop Battlefleet Gothic game this game is an adaptation of, and wrote the Gathering Storm narrative that included the Fall of Cadia and provided the plotline that was used for the prologue to this game.
As someone who follows GW's own publications I am in a better position to know what is going on, so I hope I can help. The 'Mechanicus chap' is Belisarius Cawl, a senior Archmagos of the Mechanicus who has been around for 10,000 years since the days of the Horus Heresy and is associated with Roboute Guilliman, the Primarch of the Ultramarines Space Marine chapter (Cawl created the Primaris Marines, thiough that is a spearate topic). Cawl has many areas of interest, but one is trying to understand the workings of ancient Necron technology, including the Pylons mentioned in the cutscene.
The 'scarecrow' is a Necron called Trazyn the Infinite. He is a collector of sorts, obsessed with collecting abnd catologuing artifacts and living 'specimens' - heroes and important personages from the various other species of the galaxy who he holds in status in his personal vaults at Solemnace (think Marvel's Collector, only with more advanced technology, a more sarcastic attitude, and being an uploaded alien personality in an android body).
While these two would normally be enemies (the Mechanicus considers Necrons tech-heresy abominations and the Necrons consider the Mechanicus vermin that seek to steal secrets not meant for them), here they share a common goal - neither wants the Eye of Terror to consume the galaxy. Cawl still doesn't understand the Pylon's true nature, but Trazyn was there when they were built and is able to use them to drive back the expanding Eye of Terror. Indeed, the whole reason behind Abaddon's actions over the last several decades before the events in the prologue was to destroy the Pylons in order to remove them as an obstacle to the spread of the Eye and the dominance of the Ruinous Powers.
I hope that helps.
@greg_g thanks for taking the time to explain it all, I have read some of the horus heresy books but didn't know any of what you explained. I remember the cadian shock troops as being generic guardsmen on the board game.
Geedubs - what an odd nickname, assume its come from our American cousins!
abaddon didnt destroy the other pylons in the last few decades
it was the purpose of the previous black crusades
there are a few exceptions, like the crusade he started to get his sword but most others where to destroy a pylon in the confusion
the assumption that his last 12 where failures was wrong (actually retconned but whatever)
@ashardalon You are right on that - in the recent retconn we learn that Abaddon's plan has been unfolding over millennia rather than decades, and as you say the earlier Black Crusades only appeared to fail, all actually being part of a cunning plan to render the galaxy vulnerable through destroying Pylons... that and GW got tired of people calling him 'Fail-baddon'.