A/N: Melkor was imprisoned in Mandos for 300 years before his release.
I know I say Mairon instead of Sauron a lot, but to be fair that was what his enemies called him, rather than Mairon himself. Considering how he called himself "Tar-Mairon" even in much later Ages I would guess his name being Sauron wasn't his idea and not really something he went along with.
Everything was so much quieter in Angband with Melkor gone. In nearly three hundred years Mairon had still not quite gotten used to it yet. Back in Utumno the wind howled ceaselessly for days and days on end, liable to drive someone mad if they didn't learn to tune the noise out. When he commanded Angband before the War for the Sake of Elves he only felt a fraction of the winds, but he could always tell when his master was approaching because they would become steadily louder as time passed, as if Melkor brought the maelstrom with him wherever he went. At first it had irritated Mairon, one could barely think with all of that shrieking going on outside, his only refuge was his forge nestled deep in the heart of the fortress, far enough that no sounds from the outside could penetrate its thick walls.
And now, with his master gone and the wind with him, Mairon had nothing but time to think and silence to think in.
He would have given anything, anything to have Melkor back with him. He missed the deep timber of his voice that would make the walls tremble with each syllable, missed the quake of his footsteps hunting for him, missed those eyes staring down at him shining with silver fire. He missed every single tiny thing Melkor did just to infuriate him; surprise him in the forge while he was working, mess with his tools and put them back in improper places until Mairon found them again, endlessly bother him with increasingly wild and unrealistic schemes of grandeur, drag him into an empty room where his hand would fist into Mairon's hair and his tongue would force its way past his lips...
He buried his face into his hands, his hair falling across all sides of his face like curtains of fire, and tried not to scream. He wanted it all back, Melkor, his touch, his taste, his annoying hovering, even the wind. He wanted the Vala back so much that it created an ache that gnawed endlessly at his heart, no matter how hard he tried to fight it off by immersing himself in work. Mairon bred the orcs, tended the forges and even began smithing himself, countless armors and blades pouring from his forge. He repaired the fortress and took care of Draugluin, fixed all of the rooms and bedchambers.
All under the pretense of getting everything back in order, he told himself. Except for that one, wakened part of himself that whispered he was only doing this in the hopes that Melkor would come back and everything would go back to normal.
In the silences between his work, his heart ached with its wounds.
Before Melkor had been dragged away in chains, Mairon had occasionally commanded Angband with his physical presence, leaving the Vala behind in Utumno. They were never separate for long, and Mairon could have easily had his master right there with him by lifting his voice into song like he did when he created the world with the Ainur. The voice of the Maia would roll over the northern plains of Middle-earth, unfurling like clouds of smoke and light as he sang for a duet and waited hopefully for his master to answer. When the Dark Lord's own song reached back to him, oily and rolling across the air like poison, it always filled his heart.
But now...Melkor was silent. Melkor had been silent for three hundreds years, but that never stopped Mairon's feet from finding their way to the top of the fortress to overlook their icy surroundings that stretched as far as the eye could see. And he would still lift up his voice and sing, sing desperately, every note tinged with misery as he called and called for a lord that would not answer.
All of northern Middle-earth could hear his lamentations on the rocks. The air was tinged with notes of his suffering. And yet he sang again and again, hoping beyond hope his master would one day join in and recreate their duet anew. The years dragged on and on, dragging Mairon's hope with it on days which seemed the darkest, only to be burned away by the Maia's pure devotion to his absent master each time, his spirits rising and dying and rising again like a phoenix from its ashes. Each time his voice would become louder, more bold to reach Aman and reveal his presence, more desolate as if it could draw Melkor to him like a beacon. But the world was empty, the sky hollow, and Melkor's voice was silent. The Vala did not return.
Until one day, he did.
Sauron had not even been expecting it. He had been nowhere near the roof or window where he usually sang, instead preoccupied with some other altogether trivial task that was nonetheless vital to keeping Angband running. Busy work to keep his heart and mind from gnawing themselves to pieces and away from the thought that Melkor was not here with him.
It hit then, a sound more terrible and overwhelming than anything Mairon could have ever even dreamed of, tearing through the air like a forgotten echo coming back to it origin a hundredfold stronger. It shook the bones of the earth and nearly threw him flat on his face with the force of it, the walls cracking under the strain of the noise, and filled his heart with fear. He knew that voice; he had never, ever heard it so loud in anything other than a shout of rage, but the timber and pitch were utterly unmistakable.
Melkor. Melkor was free, he had returned. But something terrible was happening, something incomprehensibly terrible. That was no cry of victory, or of summons, or of wrath, or of any of the things Mairon was familiar with from his master. It was pain, it was a terror so deep that it sank into the heart of every stone of the land. Melkor was in grave danger.
The Valar? Elves? It did not matter, Mairon barely gave it more than a momentary thought, his master was in trouble and he would sooner see himself destroyed defending him than to live with the rest of his short life knowing he had failed to do everything in his power to protect him.
The echoes of Melkor's scream were still racing across the fortress when he shoved himself to his feet, his light and brilliance blazing forth from his body in a way that had not been lit for the past three hundred years. It scorched the hall, blackening the stones and leaving the scent of ash in the air. Fury propelled him and he reached out his thoughts, burrowing his will and awareness into the tunnels deep below Angband where the balrogs slumbered, awaiting their master's return. They were awake now, roused into action by the scream they heard as well.
They would fly to help, but Sauron would not stand idle when they could move faster. "Useless whelps!" he screamed with both his mind and his voice, adding a counterpoint of trembling to Angband with Melkor's scream still an undertone. "Do you not hear our master is in danger?! Do you not hear him call? WHY ARE YOU STILL HERE?!"
The ground beneath his feet shook and he threw himself from the window, his form shifting into a strange being of light and flame that soared above the fortress on blazing wings. Taking on a specific form would require too much time and thought and Mairon found that much more vague forms did a far better job of inspiring awe and fear than anything else. Below he saw flame belch from the doors and entrances to Angband as the balrogs poured from its depths, smoke and angered fire that took flight on dreaded wings, leaving smoldering ashes in their wake. And Mairon turned, leading the charge, and propelled himself in the direction he had felt the scream coming from: west.
He saw the thing long before he actually came close enough to touch it. A horrid, bloated evil that oozed such a darkness that not even Mairon could see into its depths, all of it taking the shape of a spider. It could easily crush half of Angband if it wanted too, but its attention seemed focused on something in particular. Mairon followed it with his eyes as he approached, sharper, smaller details coming to his attention, such as the ink-black webs the spider spun and the tiny, writhing thing she was busy wrapping in them. He could see a light, a brilliant speck that even from this distance hurt his eyes, and the black fingers clutched around it. He followed the fingers up to an arm, to a form so wrapped in armor and webs he could not see clearly, but the flash of dark hair, the color of a stormy night ocean, he could recognize anywhere.
A scream of rage erupted from him and he dove down, a speck taking on a mountain, but he did not care. It had his master, he was in pain and—heat rushed by him from the whip of a balrog as the beasts descended upon the spider, hoardes of their smoke and fire choking the air as they surrounded the creature, lashing it with their whips until it screamed it pain and reared. Mairon knew they would handle the thing, only the Valar could stand up to an army of balrogs and survive. Instead he flew down in a blaze of glory, taking his normal shape only when the ground came rushing to meet him, stumbling forward on unsteady, half-formed legs as he rushed to Melkor's side.
"Master!" his voice came out far too hoarse and quiet as he beheld the sight of his Vala, so trapped in webs and unable to move, the darkness strangling him. "Hold on, my Lord, I shall free you." He drew his sword, hands barely trembling, and set it aflame with a thought. Then he cut the webs around Melkor's throat, taking the greatest care not to accidentally strike his master.
Thankfully the webs burned and parted at the touch of his blade, for Mairon had absolutely no clue what he had been going to do if they provided to be resilient to such measures. The moment he heard Melkor gasp in the air that had been robbed from his lungs he went to work on the rest of him, hacking away the other restraints on his body and ripping away leftover strands that dissolved into soot in his hands. The spider was retreating, he was dimly aware, fleeing from the balrogs and their burning flame and the light that the flames brought. Perhaps they could kill it before it got too far? It would make a spectacular trophy—
"To me, my balrogs," he felt the air shake again from Melkor's voice, its bass reaching deep into his bones and shaking him to his core in that way he missed with all of his soul. Mairon closed his eyes and savored the feeling, holding it close to him and letting the thought sink into him: everything was alright now.
A moment later he understood Melkor's words and his eyes flew open in shock. "Master?" he whispered. "Do you not wish the evil to be slain and brought to your feet?"
His master's gaze turned to him, so missed, so fierce and all at once Mairon felt that empty place inside of him fill with light once more. "Mairon," Melkor whispered, his uninjured hand coming up to touch his face, as if checking if he was real. "Help me up, Mairon," came the next order, his imperious tone back from the nuance that had taken over it earlier. "Before they come back."
Mairon leaped to obey, taking his master's hand and pulling him up, worrying gnawing at his heart that Melkor needed assistance. Had the spider truly injured him so greatly? "The spider—"
"Leave her," Melkor said, taking his hand away once he was on his feet and drawing himself up, appearing as regal and dignified as while the balrogs landed amongst them, waiting for orders. "Angband, is it close, Mairon?"
"Yes, my Lord. It is just this way," Mairon said, eager to take his master back so he could make sure his injuries were not severe.
His heart blazed and his voice wanted to burst forth from his throat into a glorious, victorious song to announce to the world that he had won. Melkor was back, his master was home again.
A/N: I know Tolkien doesn't mention Sauron rescuing Melkor with the balrogs, but honestly? Considering he had been repairing Angband in the meantime and just waiting for Melkor to return in general, I don't see why he wouldn't be there helping them since he has absolutely no reason not to.