Authors Note:

The Silmarillion has little to say of Mairon, who later became Sauron and Turingwethil. Considering these two become the Lieutenant and Herald of Melkor respectively that seems an egregious lapse to me. In the Book of Lost Tales however there is a curious reference to a pair of Valar who, unlike their peaceful constituents, take their delight in strife and warfare. These are Makar and Meássë. They alone of all the Valar craft their own house ( where the rest of the Valar utilize Aüle's skill ). Furthermore they are well known in the forging of weapons. Tulkas makes forays to their estate to keep himself in top shape where his personality of striking only when provoked is contrasted with Makar's temper of striking when he feels like it. ( sounding familiar? ) and where Tulkas uses only gauntlets of steel Makar uses a bladed weapon - unique in Valinor at the time. {where Aüle uses a forge-hammer, Manwë uses a bow, etc.}

That the home is built of Iron and is placed close to Mandos ( which in this text is a region, not a hall ) is also telling. Last of all, they are cited thus; "-they loved the unbridled turmoils which Melko [Melkor] roused through the world." and elsewhere it states that they were two of the Ainur who attuned their music to Melkor from the first.
Some extrapolations are mine (neither Meássë or Túringwethil were ever mentioned as being maia of Mandos for example). And though Makar and
Meássë were Valar, here Mairon and Turingwethil are portayed as maia - as in the Silmarillion and LOTR

Later, they are dropped from the mythology under these names but I do believe they may have been the prototypes for Mairon and Túringwethil. Under that line of thought does this story occur.

When the last of the grand estates of the Valar had been crafted it came into the hearts of not a few among the Maia to have homes of their own. This was granted to them; and many chose to build their own places near the abodes of the Valar they served.

Mairon; however, did not. For loyalty he felt for Aüle but he also did not wish to be separated from Túringwethil. Nor did she will to be separated from him, for even before the Music of the Ainur they had often dwelt together in Ilúvaters halls and were accustomed to another's company - and she herself was in the service of Námo in his hall of Mandos. So they discussed amongst themselves what should be done.

"You know that since we entered Ea that I have been sworn into the service of Námo." Túringwethil spoke. "For his spirit is very near to mine and of all the Valar he is most pleasing to me. Yet I know that you have aligned to Aüle and hold the forges to be precious. So then let us do this - that we may build our abode between the two and neither be further from our Lord than the other. And so Mairon went to Aüle and Túringwethil to Námo and both gave their blessing - saying "You may build where you please, for there is no place in Valinor restricted to you."

And so gathering any of the Maia who had not yet chosen lords; yet were fond of striving and dark things, they made for themselves a great hall.

Of this hall they were exceedingly proud.

Here it was that Túringwethil first saw the fluttering shapes of bats beneath the glowing light of Telperion's outstretched limbs. And she saw them nest amid the white blossoms and at once fell in love with them; though many others had little love for these creatures. For her part Yavanna was at once glad and bitter.

"For I love all the creatures of the world, yet some more and some less; for these creatures were created where my thoughts and Melkors met and my love for them is little. But I am glad that there is a protectress for them."

And Túringwethil was glad of them. But Mairon was fond of the great wolves that lurked amid the mountain passes and often would he roam among them in those early days. Many of the Ainur despaired of these two and held great dread for them. For it was not forgotten by any in Valinor that when discord had entered into Ilúvaters theme at the instigation of Melkor that of the Ainur who had attuned themselves to Melkors theme that Mairon and Túringwethil had sang loudest - and for this they were shunned among their kin.

But they were happy in their abode. And at night when the days training had ended and all weariness had faded and all hurts been salved they would sit in a great iron hall hung with shields and furs. And Mairon would sit in his seat with his sword, but Túringwethil kept nearby her spear, and the warriors in their hall would sing songs of the great battles they had fought and make promises of great deeds to come. There in the hall would they drink and eat and be merry and upon the morn they would rouse themselves again and clad in bright mail they would battle one another once more with a fury so great that none would suspect the warriors trading blows in the morning would drink together come the evening.

Now as it has been said before, that Túringwethil was a maia of Námo. Her task within the halls of Mandos, the fortress of Vé, that she tend to those residing within according to their Doom. Yet; only one there was in those earliest days before the come of the Children of Ilúvater - and that was Melkor after he was returned to Valinor bound in Angainor.

As for that event, Mairon himself had a part. for when it was decided to subdue Melkor for the sake of the coming of the Firstborn it was he himself who made for them their bright armor - for Aüle was busied with the task of creating a chain and fetters with which to hold the mightiest of Ainur ( and this was no small feat). The finest of armors he crafted, in silver and gold and studded with fine jems and wrought with great cunning and spells of strength and endurance.

And though Mairon was disheartened that Melkor should be constrained he was nonetheless happy to instruct the Valar in all manner of warfare - of which he and Túringwethil had great knowledge. Yet; she for her part was busy relaying messages among the Ainur.

Yet; for all their aid Mairon and Túringwethil went not when the Ainur left to make war with Melkor. But upon their victorious return they went with all others to the court that had been made before Taniquetil. Sat in a chair of silver Manwë presided over all and would hear any claims for or against Melkor that could be made - before the Doom of Melkor should be spoken.

And many voices were against Melkor on that day - for there were few among the Ainur who had not a grievance against him for some work destroyed. Chief among these were Aüle and Yavanna who lost much of their hard labor to the capricious nature of Melkor's malice.

And when all voices had died away Manwë asked:

"And who here might speak in defense of Melkor? For this is offered before judgment is passed."

And then it was that Mairon spoke up, though he himself had as of yet no great love for Melkor - yet he was displeased with the unceasing tranquility of Valinor.

In the end it was the for tree ages Melkor was doomed to be bound within Námo's halls as prisoner in the fortress of the Valar. Yet after he was to spend four ages as a servant in the house of Tulkas.

Now, while many among the Ainur thought this exceedingly merciful it seemed to Mairon to be most severe. For he spoke later -

"Did not Ilúvater tell them with his own words? That the beauty they would create is but in many cases magnified through the work of Melkor? Though his intent be malicious his works redound in the end to increase the glory of those he would slight. They all agreed to this end, yet how quickly they have forgotten. Yet they would have for us all a life of eternal quiet, of unending monotony without strife or glory. The thought itself is almost unbearable and enough to make one wish they had never come to Ea at all!"

But Túringwethil simply looked on silently, as was her wont, and kept her thoughts to herself. Yet; even she at length spoke and her voice was quiet.

"I fear now that Manwë for his wisdom does not truly know the hearts of others. For if I were to foresee the future even I could know that Melkor's servitude under Tulkas itself may be enough to drive him to madness - for I would not chance that Tulkas would be much in mind to treat him kindly or stay his hand in anger, and such treatment in the end will only serve to embitter one who already believes themselves wronged."

And Mairon much agreed with her and a sense of foreboding was in his heart. When such heavy thoughts became too much he at last went to Aüle and spoke to him; yet his master was most unsympathetic.

"It is a harsh thing yes, but in the end Melkor must be made to see how he has erred and thus endeavor to win back the favor of his kin."

And Mairon replied; "What kin will he consider any who would punish him so? For what has he done beyond the destructiveness of a child? Or has he raised a hand against the Ainur in threat before his capture at Utumno? Yet twice has he been struck and he alone among the Ainur has known the pain of a strike - for affronting his fellow Valar and destroying their works. Yet he has not harmed them. No, in fact the first violence of one Ainur toward another was first struck by Tulkas and without provocation save for the strong Valas anger. And for what cause? For though Ilúvater allowed Melkor's entry into Arda; knowing truly who Melkor is in his heart - yet He did not hinder him. But did Manwë not invoke Ilúvater as his reason for constraining Melkor; that Melkor may not hinder Iluvater's design? Yet how can that be if Ilúvater himself did not prevent his entry into this world - which means that Melkors actions then are part of Ilúvaters great plan. So will the Valar still accost him for this?"

And this Aüle could not answer - because for all his malice and destruction as of yet Melkor had not caused harm to another Ainur and the Children of Ilúvater were not yet born that they may be tormented by him. But he could not abide that a maia of his own service could speak on behalf of the abhorrent one, and so he commanded of Mairon.

"Be gone from my house or be still! For this is a matter dear to the hearts of all - that their works remain un-destroyed and that strife remain far from their homes. Twice already have you spoken for Melkor and alone at that. I will not suffer a third time."

And at this Mairon fell silent and went from the house of Aüle in fury. Even still, he was not ready to forgo his Lord yet, though he disagreed deeply with him at heart.