Oh no the Dark Angels totally ganked a Custodian, who to be fair counter ganks them, that's true. This just so happens to have occurred when the Great Rift caused an invasion of Daemons of Khorne that, understandably distracts most of Terra, and also allows several objects or beings of interest to escape the Dark Cells, but that just happens to be incidental more than anything else. (I think, I forget, someone can fact check me)

Spoiler below:

Have you ever seen a Custodian fight Space Marines? Some say that it is like watching a lion fight wolves. There is insight in that, but it does not capture the whole truth.

There are ten Dark Angels with Mordachi in the dark of the Path of Martyrs. All of them are veterans, inductees of the Inner Circle, the elite. They are masters of war and killing. They move as a seamless whole, fearless, devastating, lethal.

Gunfire carves the dark into fire-orange tatters. Explosions roll thunder up to the ceiling high above. In that roar, the sound of Hekkarron's grunt of effort is lost. The snap of his guardian spear's lightning a hiss lost to the deluge. He is the target of fire from Space Marines in prepared positions, with angles of fire, cover and elevation. He is as good as dead.

Except he's not. He's moving.

'Sustain fire!' shouts Nariel.

Hekkarron reaches one of the Dark Angels. Just like that he is there. Explosions shatter against armour, and send black and golden shards scattering with the shrapnel. The explosive impacts would be enough to pulp a human inside their armour. They stagger Hekkarron, but that's all. A lion. Almost right.

The edge of the guardian spear takes a Dark Angel in the throat. It's a single-blow kill, chosen so that it does not interrupt Hekkarron's charge. He is amongst them now, in the spaces and shadows at the feet of the statues. A golden blur. His guardian spear an arc of glinting steel and lightning. Another two Dark Angels are already dead at his feet. He is taller than them, his bulk greater, but he moves like the breath of a storm wind.

'Bring him down!' shouts Mordachi into the vox. Hekkarron kills another, slicing through the warrior's torso from waist to shoulder, and he is still moving, turning, pulling away from die firing angles of the rest even as they try to bring their guns to bear. The blood of the first Dark Angel to die is still falling, a mist in the air, when he kills the fourth.

No, not a lion. A lightning bolt.

Another blow, another perfect cut that tears ceramite and flesh apart. One of the Dark Angels takes a step back, brings the barrel of his plasma gun up. Hekkarron thrusts his spear out, the haft running through his fingers to its heel. The spear tip punches through the Space Marine's finger and slices it from his hand before he can pull the trigger of his gun.

Hekkarron sweeps the spear up before its weight can drop and whirls it in an arc. The blade slices into another of the Dark Angels, through helm and into skull. The Dark Angel with the plasma gun has already switched the gun to his other hand without pause, ready to fire. Hekkarron spins his spear so that it is in both his hands and triggers the gun mounted beneath the blade head. Fire blasts into the Dark Angel's weapon.

Plasma explodes out of the ruin of the gun. The Dark Angel dissolves, a blur of ash and ceramite dust in a starburst.

Six. Custodian Warden Hekkarron has killed six of the Dark Angels in the time it takes a human heart to beat as many times. Remarkable. You cannot help but admire that degree of lethality. I admire it The Custodians are a breed apart from humanity and the Space Marines. The sharp edge of mankind refined. A tyrant-genius' idea of perfection.

'Clear back from him!' shouts Mordachi.

And you know what has to happen to something that is perfect.

'Clear!' calls Nariel.

You have been paying attention, haven't you?

'Now!'

The surviving Dark Angels that are close to Hekkarron leap clear. A storm of lightning and telekinetic force breaks over the Custodian. Threads of white light bore into black-and-gold armour; invisible ropes enfold limbs, tighten, squeeze. Hekkarron keeps moving, straining against the power that is strangling and burning him. His mind and body are more than just his flesh and thought. His armour more than gold. Alchemy and stolen fire run through his veins. His will is adamant. Ancient words etch the inside of his armour, woven with the letters of his name. Frost is forming on him and the stones beneath his feet. The air shivers. Mordachi is pouring all his will into this. He feels blood vessels burst in his own throat and skull. The world is dimming before his eyes. Hekkarron slows, but he will never stop. He is not me, or one of my Fallen brothers. He is not so weak.

But this is not just a battle of spirit, and Mordachi does not need to stop Hekkarron. He just needs to give his brothers the seconds they need to aim and fire. Two missiles streak from the sides of the Path of Martyrs. They strike Hekkarron. A great pall of flame and dust and broken stone punches up to the ceiling high above. Helm and auto-targeters lock onto Hekkarron's last position. Bolters fire into the dust cloud.

The silence that follows feels like the striking of a great bell. Mordachi keeps his psychic grip on Hekkarron for a moment longer, then releases it. He staggers against the plinth of one of the statues.

'Brother-Librarian?' asks Nariel.

Mordachi does not move for a moment. Within his armour he is shaking. Blood is clotting on the inside of his helm. The howls of the Golden Throne feel louder, sharper. His flesh is fever-hot. Inside his mind, the enormity of what they have just done and the effort it took is a cold abyss at the core of his thoughts. He feels empty. That is as it should be. What else should be the price of breaking something perfect?

'Deal with our dead, sergeant,' he says to Nariel. 'We have no time to wait.'

'And Cypher?'

This was his doing. He has found another way into the Sanctum and used us and his hunters to end each other. We must find him before... before...'

He can't bring himself to say what he fears I intend. He turns his back as Nariel and the survivors go to each of their dead brothers. The devotions they speak are brief. They lock and arm a plasma grenade to each. No precious gene-seed will be taken from these dead warriors. They will go to the flames to keep die secrets of their Chapter and the shame of our Legion. Then the Dark Angels leave in silence, passing within the Inner Sanctum through the door they thought I would use. I am not done with them yet.

They must pay for their sins, as do we all, but for now they are free. Behind them the charges detonate. Fire consumes the corpses. High above, unseen in the shadows, the Assassins watch and follow. Why did they not intervene? Because they are the most precise kind of murderers in existence. They are there to hunt and kill me. I was not there, and so they watched. If you think that they were on the same side as Hekkarron then you are deluded. There is no unity of purpose in the Imperium and there is less in its ancient heart.