system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K/WoW] A wire gets crossed and instead of the Dark Portal opening to Draenor, it opens to Macragge.

And for the sake of argument lets just say the Warhammer side is just a regular portal hanging in the air and not an elaborate structure like the Dark Portal itself is. Also this takes place just before Legion, for whatever reason.

How do the Ultramarines handle a planet that is CLEARLY drenched in the Ruinous Powers of Chaos?

How do the denizens of Azeroth handle technology and faith they've never even conceived of?
gpt
Azeroth get a purging, the planet is completely overwhelmed by Xenos.  
There is yet hope for the planet however, it is very clear the planet is blessed by the Emperor. The humans that do make their living there might be tainted but they show power that is clearly miracles by the Emperor in one of his many forms, he is the guiding light of this world.  
Also, abhumans? And some Humans are cursed? Even allied with some Xenos? Are those Eldar?

There is hope for this world but a great crusade to purge it must commence. Thankfully, the planet appears to be in the most early stage of industrialization with minimal amount of military vehicles. They run on steam even.  
The unfortunate part is the fact there is a great amount of psykers on the world.

The PDF of Macragge is organized in to an invasion force while the local Space Marines starts to scout out the planet so they can perform deep strikes on enemy command structure, this "Horde" faction is by far the largest and most dangerous and thus, their cities are bombarded by Thunderhawks.

Some Space Marines commanders and Imperial Diplomat alongside priests are sent to the Human capital city, they will be explained that they have finally been found, the Emperors guiding light have been with them all this time but the time to cleanse the enemies of man is on us.  
Why is this human king, who is clearly a warrior and general, so keen on preserving some of the Xenos? Clearly, this world, while still have hope, it is a frail and weak hope, it must be reinforced by fanatic zeal.