didn't custodes immediately open fire on Magnus when he came thru, and because his form was engulfed with flames making a light sou bright that it blinded mortals looking at him. All the while generators were exploding killing humans left and right while klaxons whine.

10017667
Pretty close: Thousand Sons said that the Custodes were 'at arms' when the alarms went off, but never said anything about opening fire. That was at the initial Warp Rift in the genetics lab when 'they' stole the primarchs. They just kinda saw Magnus (or what they saw of him, rather than what he saw of himself) and that was kinda it. Magnus blanks it, realizes what happens, aaaaand left without telling the Emperor what happened.

For such a surprisingly important event, it's scary short with like 4 lines total, and I can't repeat it verbatim because, well, plagiarism . Plus, I kinda wanted to add just a small little addition that'll be important in the future... it also toys with the Master of Mankind (and kinda Vengeful Spirit) ethos that the Emperor appears as the people around him want him to appear (Arkhan Land, etc).

That tree...that was the Tree of Harmony, wasn't it.
And the brief flash of an 8 branch weed...the Four have set their sights on Equestria, haven't they...