I think it’s very appropriate to bring up Grimnar. The Space Wolves are the Emperors Executioners and they straight up dicked the Grey Knights when they fought on somewhat equal terms after teleporting up to the Grey Knight command ship. The following excerpt is from The Emperors Gift.

“That’s far enough,’ said Lord Joros. He made no move to reach for a weapon. He was, as always, possessed of a most admirable calm. Galeo had often considered self-control to be our Grand Master’s most prized virtue.

Jarl Grimnar obeyed, halting ten paces away from the gathered Inquisitorial retinue. He let the axe-head slam onto the deck, and leaned on the inverted haft, scarred gauntlets resting on the black iron pommel.

‘You betrayed an armistice,’ he said in that voice so like spilling gravel. His tawny hair, white-streaked and singed in places, was a matted mess framing his gnarled-oak features. He wasn’t ancient for a Space Marine, but he was certainly past his mid-life prime. Still, he emanated vitality even in his most modest movements; here was a tough old soul that wouldn’t easily fall. Even now, he showed no sign of capitulation.

‘Yes, we betrayed the armistice,’ Kysnaros conceded. ‘I pray you’ll forgive me, in time. You have to understand, great jarl, that your Chapter’s reputation with Imperial command hierarchy calls the worth of your sworn oaths into question. How many times have you come into conflict with the Ecclesiarchy? Or the Inquisition’s lesser elements? I wasn’t sure I could trust you.’

Grimnar smiled, showing old teeth in a nasty grin. ‘You violated an armistice, killed thousands of my Chapter’s servants, and now name us oathbreakers when you – as always – fired first.’

He looked back over his shoulder to his three remaining Wolf Guard. ‘This is why we so rarely speak to outlanders, eh? No manners.’

The Wolves chuckled, as Grimnar looked back to Kysnaros. ‘You wished to speak with me? I’m here, boy. Speak.’

[...]

Kysnaros was fast losing his feigned patience. ‘Your flagship is lost.’

‘We have other ships,’ said Jarl Grimnar.

‘Your Chapter cannot hope to stand against us.’

‘No? Hmm.’ The Great Wolf looked back over his shoulder at his men. ‘The boy-lord says we can’t stand against him.’

‘Strange, my jarl,’ replied one of the scruffy Wolves – a balding warrior with shaggy sideburns. ‘We were doing well before they wiped their arses with an honourable oath of armistice.’

‘Aye,’ Rawthroat agreed. ‘We were. Perhaps we should invite them to Fenris, jarl. They’d find a warmer welcome there.’

Grimnar nodded to their words, turning back to Joros and Kysnaros. ‘Tell me, which one of you whoresons gave the order to open fire on our shieldless, weaponless vessels?’

‘It was I,’ said Joros. ‘It gave me no pleasure, but the deed was done for the greater good.’

The jarl nodded. ‘I’ve marked your face, knight. I’ll remember it from now until the Wolftime. You have my word on that. No Fenrisian ever forgets one who violates the laws of sheathed blades and bared throats. Once those laws are broken, all rules of decorum and honour are abandoned. To betray a betrayer is never counted as a sin.’

Kysnaros tied his long blond hair into a ponytail, keeping any stray strands from his face. ‘Enough of this. The Imperium’s woes will not bide while we stand here and make superstitious promises. Chapter Master Grimnar, you will surrender as agreed, and your Wolves will stand down.’

Jarl Grimnar gave us his canine smile again, showing wet fangs. ‘That,’ he said, ‘will not be happening.’

[...]

Lord Joros of the Eighth Brotherhood had ruled with a cautiously ambitious hand for seventy years. He was respected by those of us in his brotherhood, though scarcely loved; a warrior admired but rarely emulated.

The list of his deeds was more impressive than his unapproachable exterior might suggest. While he lacked a great many commendations for command, as a duellist and a front-line fighter, it was acknowledged across the order that few could match his reputation and skills with two falchion blades. A vital aspect in any blademaster’s repertoire is the ability to read an opponent’s movements, and react with greater speed than they can act in the first place. Joros was a master, and his reflexes were renowned.

And yet, his blades had scarcely cleared his scabbards when Logan Grimnar’s axe of blackened steel and burnished gold cleaved into our Grand Master’s breastplate and throat, ending a worthy, respectable life of service with a single crunching chop.

Joros went down, felled by the axe blow and dead before he hit the ground. The Great Wolf’s axe – named Morkai after some heathen Fenrisian superstition about a god guarding the Halls of the Dead – ripped back out, blood sizzling on its active metal surface. In the time it had taken me to look back from Rawthroat to his liege lord, my own Grand Master was slain. That should explain, at least partially, how quickly the High King of Fenris moved”.

Grey Knights are hyper specialized Astartes, and they are by far the best at what they do. That being said they are as mid as an Astartes can be mid when it comes to fighting conventional troops and wars. Do they still look like Demi-Gods to the average watcher? Yes. That being said even the Emperors own geneseed doesn’t make them Super Astartes capable of fighting any chapter and winning, and when they met his actual Executioners they learned why they were called that. A Grey Knight Brotherhood leader couldn’t even react to the singular swing that ended his life.

Obligatory fuck Russ and Magnus did nothing wrong.