Their most likely reaction?

[JMISBEST]

What's The Imperiums likely reaction to The Uber Empire sending its new Prototype TL 25 Super-Heavy 852,200 ton Carrier with 16,102 crew and 1,144 Marines, that carries 1,535 30 ton, 898 ton, 447 60 ton and 248 80 ton Fighters that also has 17 100 ton and 5 200 Ton Modular Cutterás and 11 90 ton Launch Tubes that has Prototype 5 TL 27 Antimatter Shields that no weapon below TL 16 can get through and reduces all damage from TL 16+ by half before other modifiers and A Prototype TL 27 Canno that's part Anti-Matter, Part Gauss and Part-Nuclear that can easlly destroy a minikum of 1,00 ships in 1 shot showing up in time to prevent 1 of their own renegade admirals leading a coalition of Theev, The Zhodanni, The Glorious Empire and assorted Hierate, Imperial and Floranni Rebels enslaving The Imperium within 10 years and all of known space within 32 years?

 

[Sigtrygg]

The Emperor would order the use of a secret Ancient artifact discovered in the Shionthy belt.

This is a single use reality manipulation device that is used to remove the Uber Empire to its own pocket universe from where it can never return or ever again interact with the Imperium's universe.

 

[M J Dougherty]

I'm not sure how the number of marines aboard a vessel so advanced that the entire Imperial Grand Fleet cannot harm it would matter.

This is so far outside the boundaries of the existing setting that the only anwer seems to be 'surrender, or die in droves then surrender.'

 

[M J Dougherty]

The answer to all your questions is 'your uber-empire just does whatever it pleases and the Imperium tries to survive as best it can.'

There is nothing meaningful that ants can do against the boot of a god.