system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] The Emperor is about to die in the throe, the IoM can not beat the orks, chaos or the tyranids, how does Emperor expect to win?
gpt
The Emperor exists in a horrifying state of unlife/undeath. His physical body damaged well beyond repair - his soul is tethered to our mortal realm trapped inside a withered and decarying husk. 

But humanity would not have survived this long without the Emperor's immortal sacrifice. Were his body to fail his soul would be fully freed into the warp ascending him to Godhood.

* Immediately the Astronomicon would burn out, leaving humanity mostly blind in the ways of Warp Travel.

* The Emperor would be free to take more direct action as a warp god (ass opposed to his current limited involvement). 

From here the Emperor goes two ways - either we sacrifice a large number of his immortal children - miming the shamanistic sacrifice of old to 'rebirth' him. Or he simply stays in his pure warp form and guides humanity as a true God. 

1) Call his children back - Russ, Guilliman, Vulcan, etc. He imparts his love and wisdom onto them and sets them on their respective tasks. 

* Russ leads the spearhead against the Orks counter attacking with such savagery that the initial WAAAAAGHH is stopped and even pushed back. 

* Guilliman leads the defense of many beleagured worlds in a manner not seen since the Hades Hive of Armageddon. The previously flimsy and waivering resistance is emboldened by the Primarch and quickly reforms cold and unbreaking firing lines - bringing swift disciplined fire down on the advancing greenskins. 

* Vulcan begins building great seige defenses and advances much of the mechanicus knowledge. While he has the least glamorous role - the after effects of his work will prove crucial to the survival of the Imperium - the advances in tech and preservation of knowledge will embolden the coming counter offensive. 

* The Emperor - after dictating his wishes to his sons will reactivate the Astronomicon - either throgh divine will or by teaching humanity it's secrets. 

For the time being teh Imperium is saved and the initial WAAAAAGHHH is broken. The lines have held and we have regrouped but so have the Orks. 

Emboldened by their defeat of the Tyranids the Orks descend again by the millions. Testing the Imperiums defenses and slamming into the weakest points. 

Suddenly from behind Orkish lines come Corax and the Lion on two different fronts - leading their respective chapters with untold fury. The sight of two returning Primarchs sends the defenders into a frenzy and the Orkish WAAAAGHH is shattered on two of the four fronts. 

From the Ghoul Stars... the Orks just stop. The turn away from the Imperium and attack something behind them. All the Imperium knows is that the Orks were destroyed utterly and no evidence of the Xenos that did it is left. Few in the Imperium recognize this - those that do order all available defenders to outside the Ghoul Stars to stave off the cold and impending death. 

On the final front the Orks gather for literally the largest single attack seen since the Heresy - Chaos seeing the Emperor reborn throws all it has in with the Orks. 

The whole host of just about every nasty thing descends on the Imperium. The Primarchs - recieving the blessings of the God Emperor rally the totality of their chapters and brace for the most gruesome fighting since the Heresy. 

Ynnead springs to life from the last breath of the Eldar - having been formed by a mass suicide in a last ditch gamble. Slaan'esh is destroyed utterly - destroying many advancing chaos forces and throwing the lines into dissarray. 

The Tau appear in force - not out of kindness but seeing that literally the fate of the galaxy rests on the events ahead - throw every bit of their might into the fray. 

The fighting is bloody and horrifying but the Imperium wins the day - grudgingly allowing the Tau to withdraw without further bloodshed. 

Chaos is broken but not exterminated - the Imperium rebuilds with the Primarchs leading humanity. The Emperor is praised. 

And we wait for the next Xenos threat...

Edit: Or you know something like that.