The one who had slipped into the Halls of Mandos was Morgoth.
After escaping the Void and returning to Arda, he had not announced himself. He had made no move toward open conflict. Instead he had come quietly, moving in shadow all the way to Valinor, and there he had disguised himself as a common Elf and taken up residence in the city of Valmar, watching and listening.
His disguise was extraordinary. Not only did the Elves around him detect nothing amiss, even the Valar themselves noticed no irregularity. He remained hidden in Valmar, gathering information piece by piece, patient in a way that was unlike the Morgoth the world had known.
His patience was not without cause. Shortly after arriving, he had seen the Two Trees of Valinor standing outside the western gate of the city, alive and restored. He knew what that meant. He had destroyed those Trees himself, and now they had been brought back by Sylas using the manipulation of time.
As Morgoth quietly assembled the full account of what Sylas had done, including the defeat of Ungoliant, he became more careful still, pressing himself deeper into concealment and waiting.
The creation of the Underworld was what finally gave him his opening.
He disguised himself as a newly summoned soul of a dead mortal from Middle-earth and joined the procession of the dead moving along the road toward the Halls of Mandos. The divine light within the Halls illuminated the true nature of every soul that passed through, but Morgoth's disguise held even against that scrutiny.
He passed the examination with a clean and innocent identity, moved through the Halls unchallenged, and made his way without difficulty to the Gate of Sorrow.
The Halls of Mandos served only as a waypoint for the souls of Men. Those found innocent passed directly through the Gate of Sorrow, leaving Arda behind for whatever unknown land of destiny lay beyond it. The Gate accepted only human souls.
Even the Valar could not pass through it, and no force known to Arda could break that restriction.
Morgoth's eyes held no worry. Only absolute confidence.
He walked to the Gate of Sorrow, dropped his disguise, and opened his hand.
Resting in his palm was a human soul, but unlike any other in that place. Its eyes were blank and empty, drained of all consciousness, its entire being stained through with darkness and radiating a faint evil that had become part of its substance.
This was a soul Morgoth had spent considerable time refining and remaking. It was no longer a person. It was a puppet.
He looked at it without expression and tore it apart.
From the fragments, he drew out a sliver of his own soul and pressed it into the remnants, then shaped the pieces back into a whole. What reformed in his palm was a complete and unblemished human soul, indistinguishable from any innocent dead. But its eyes were no longer empty.
They were alive, sharp, and filled with the same cold malevolence that lived behind Morgoth's own gaze.
"You know what must be done," Morgoth said.
The soul in his palm smiled, easy and dark. "Of course, my original self. I will find the Land of Destiny and seize the Secret Flame."
It launched itself from his palm, shot toward the Gate of Sorrow, and vanished through the shimmering boundary without slowing.
Morgoth felt the connection the moment it severed. The instant his soul clone crossed through the Gate, the link between them snapped clean, as though that fragment of himself had ceased to exist entirely. He stood still for a moment, looking at the place where it had gone.
He had prepared himself for this. He did not believe the soul had truly been destroyed. Something on the other side, some power greater than anything in Arda, had simply swallowed the connection whole. That was expected.
He tilted his head back and looked at the void above him, a cold smile forming on his face. "You think that hiding the Secret Flame among those wretched, lowly human souls will stop me from finding it?"
He did not speak the name of Ilúvatar aloud. He feared that naming him might draw his attention and expose the scheme before it could bear fruit.
Then dark smoke began to pour from Morgoth's body in great churning streams, and from that smoke came soul after soul after soul, tens of thousands of them, each one identical in form to the puppet he had just sent through.
These were the souls of lives that Sauron had once sacrificed to Morgoth in dark ritual, devoured and absorbed into him over long ages, now pulled back out of him and given shape.
Morgoth's brow tightened slightly. He fragmented hundreds of thousands of shards of his own soul and pressed one into each of those hollow human forms, making them his clones. The cost was visible. His aura destabilized, his power diminished noticeably. The scale of self-fragmentation required for this was not trivial, even for him.
He had decided it was worth it.
With a single gesture, he sent them all through the Gate of Sorrow.
One by one they crossed the boundary. One by one the connections severed, each clone vanishing beyond the Gate into silence, leaving no trace Morgoth could feel or follow.
He was not troubled by this. He was patient now in a way he had not been in the elder days, and he settled in to wait.
And so he waited in silence.
Time passed. The Underworld completed its synchronization with the rest of Arda, and the gathering of Valar began to make their way back toward Valinor. Morgoth's expression did not change. He showed no concern at all about being found.
Then his eyes sharpened, and he fixed his gaze on the Gate of Sorrow.
The Gate, which had stood perfectly still and impassive through all of it, began to ripple. Behind its shimmering, mercury-like surface, disturbances moved and grew, as though something on the other side was pressing against the boundary, trying to push through from beyond.
Morgoth's eyes lit up.
Without a moment's hesitation, he unleashed the full force of his power against the Gate of Sorrow.
The attack was deafening. The impact was catastrophic. The ancient walls of the Halls of Mandos, built to endure beyond the reckoning of any age, shattered under the force of it, crumbling and collapsing and leveling to the ground.
At the same moment, across the distance, Mandos felt it.
His expression shifted completely. He turned toward Valinor with sudden urgency. "Something is wrong. The Halls are under attack."
He did not finish the words before he was already moving, teleporting toward Valinor at once. The other Valar looked to one another, unease spreading visibly across their faces. They had all felt the shockwave of that power rolling out across the world.
Manwë's expression went cold. His voice dropped. "That is Morgoth. He has broken his chains and returned."
As Morgoth's brother, Manwë knew that power with perfect certainty, recognized it the way one recognizes a voice heard since before time was counted. The other Valar heard him and their faces grew grave. They understood what Morgoth represented better than anyone.
The only reason they had defeated him before was that Morgoth, in his hunger to dominate everything, had spread his power so thin across the world that he had weakened himself in the attempt. Now he had returned from the Void with that power restored, perhaps even greater than it had been.
The weight of that settled over all of them.
Manwë turned to Sylas directly. "Sylas, even at our full speed we cannot stop Morgoth in time. Only your power over time can turn this. I am asking you, please, stop him."
"I understand," Sylas said. The situation needed no further explanation.
He reached into the current of time and stepped into the river of time, moving swiftly upstream, intending to go back far enough to cut the problem out at its root before Morgoth's plan could unfold. But a heartbeat later the river lurched.
Something seized the current around him and expelled him forcibly, squeezing him out of the flow and dropping him squarely into the present moment, at the exact point in time where Morgoth stood assaulting the Gate of Sorrow.
Morgoth looked at Sylas's expression of surprise and let out a slow, contemptuous smile.
"Did I not warn you? I can draw upon a portion of every law and power that exists in this world." He let that land. "Your power over time is no exception."
This was Morgoth's singular nature as the mightiest being Ilúvatar had ever made. Other beings held specific authorities granted to them. Morgoth held something broader and stranger: the right to reach into any law, any power, any force operating in the world, and take a portion of it for his own use.
The sole exception was the Secret Flame of Ilúvatar himself, that power which belonged to no created thing and could not be borrowed or stolen. Everything else was available to him.
And crucially, even the being who held a particular power as their own could not revoke his right to use a portion of it. That was the nature of what Ilúvatar had given him at his making: an authority that could not be stripped away, a favoritism written into the foundation of his existence.
So when Sylas had entered the river of time, Morgoth had sensed it immediately, reached into that same current, and used just enough of its power to prevent Sylas from traveling further into the past.