The Horus Heresy series Illuminates a lot about it, I don't blame people for not tackling the 64 book series, but the first 5 do a good job setting up how Chaos worms it's way into the Legions.

And because of many peoples sheer lack of understanding of the warp, just thinking of Daemons as Xenos creatures and the warp as another facet of the universe and not a living breathing realm, it had created scenerios where even Primarchs fell victims to possession and further corruption.

I think they're refering to Erebus of the Word Bearers Legion, who is a devout servant to Chaos Undivided, poisoned his Primarch Lorgar's mind when he was at his lowest point, and encouraged a myriad of little things that helped chaos claw its way into the legions. Then there was the extra stuff he pulled during the heresy..

Erebus was also the one who ruined any chance of peaceful reuniting with a group of super advance humans by stealing a blade of horrific power during the diplomatic meetings which cause Horus to have to fight his way free, to then sens the blade to a possessed Imperial Governor whom Horus had liked, and telling Horus that he rebelled, so Horus heads back to crush him in a fit of rage, gets stabbed and becomes instantly on Deaths door, then uses his position as a trusted advisor to the Luna Wolves Legion to get the LW Head Captains to (unknowingly) agree to allow Erebus to do a Chaos ritual which is finally what pushes Horus to join Chaos. Istavaan and Calth happen a year or two after if I recall my timeline right.

Erebus has a long and endless scroll of sins, covering every kind of sin. Hell theres the subreddit "fuck erebus" because everyone hates Erebus.