Note: the Great Declaration is not just able to happen in God!Cain universe, it can also happen in regular Warmaster universe as well. Karamazov's Madness is quite flexible in that regards, and it doesn't discriminate based on past history of loyalty. Well, it does, in the fact that if you have a history of angering the Inquisition, it goes without saying that you are more likely to be targeted. In any case, HERE'S TARKIN!
The Inquisition's Missive
Inquisitor K. Tirkan was in a good mood today, in as good a mood as an Inquisitor of the Ordo Hereticus can be. The Holy Ordos was finally taking note of the sheer magnitude of Cain's heresy, and a true hero had been tasked with leading the charge. The price would be steep, and has already proven so even after a few days. The heretics were revealing themselves left and right, just as planned by High Lord Inquisitor Kiln before he secreted himself and their allies away on the Eisenstein's Legacy. That, and the other plan that Kiln had gave him the task of completing. The plan of removing what had long been a thorn in the side of the Holy Ordos, and said thorn had now been found to be heretical in a way only the Mutant and the Heretic could be.
The Space Wolves were just found to be heretically deviant on level equivalent to or more than that of the Blood Angels, but couldn't be dealt with publicly.
So, Tirkan was to deliver a message to the Space Wolves: Chain yourselves to the Holy Ordos now and forever, or be exterminated like the Thousand Sons for your mutant ways.
Tirkan sorely wanted to use virus bombs, but then an idea came to him in a dream. What if one could exterminatus a planet through extreme cold? And so, the Fimbelwinter bomb was born from the darkest recesses of his subconscious, able to to render a planetary atmosphere frozen solid in mere seconds. He didn't know where the name came from, it just appeared to him in a dream, but who cared? The Ordos had another tool in its arsenal.
Of course, the Fimbelwinter was only to be used by Tirkan if they didn't vow to hunt down the Blood Angels, Ultramarines, and their successor chapters. Oh, and the White Scars, Raven Guard, Salamanders, and Imperial Fists if said chapters impeded the Holy Ordos in any way.
In any case, he would stay here with the Fimbelwinter as the Space Wolves helped hunt down the foes of the Imperium, to perhaps even including the Celestial Lions if they spoke up again. That was just Tirkan's opinion though, he didn't know if Kiln would authorize it. Tirkan hoped Lord Kiln would, in all honesty. But, in order for that to happen, some...encouragement was needed to motivate the Space Wolves to aid the Ordos. Hence taking Fenris hostage.
Tirkan would give the Space Wolves ten(10) minutes to comply with his demands, and if the Space Wolves were not on Fenris, he would blow it up, as it would be obvious that the Space Wolves had decided to raid the nearest planet, such are the ways of the Mutant. Or they were going to Terra in a vain attempt to overthrow Kiln, as the Heretic would, which would also see Fenris needing to be blown up so as to stop the heresy.
May the God Emperor bless this holy work, Tirkan thought, as he approached Fenris with a smile on his face. Fimbulwinter at hand, and the Mutant would learn to the Emperor once more!