The Unlikely Visitor

Chapter 21.
In which there is black (speech) mail, and a looking glass is employed as emergency exit.

Ginny didn’t see Sauron again. He did keep his promise though. One day that summer she came into her room and found on her bed the spear with which she had made her way through the labyrinth, and a familiar looking sketchpad, accompanied by a letter in elegant Tengwar. It took quite a bit of translating, but the Maia had been so thoughtful to add the English translation of words she didn’t know in the margin. It made her smirk… “Thoughtful” and “Sauron” still made for an odd combination in her mind.

Ginny,

A warrior’s word is his bond, so I’ve kept my promise. I returned your weapon too, just in case you would ever come across a couple bloodthirsty dwarves again. Staking your brothers on it can be a possible use as well. I also left you my drawings, since you liked them for some reason and I have no further use for them anyway.
They found a way to send me back to Middle-Earth, using an artifact called the Mirror of Erised. It usually shows you what you want most, but apparently it can also take you to where you want to be most, with the right rituals. I don’t feel like explaining it. Either way, if everything goes right I will not be here anymore tomorrow. You will also be getting another astronomy teacher.
I spent only one year in your world, but I learned a lot. Even from you, young, annoying and inexperienced as you might be. I hope you will remember the things I in turn tried to teach you, and then I don’t just mean the evil-sounding expletives.
We won’t meet again, but you will forever live in my memory.

Sincerely,
Sauron Gorthaur

Fred and George also received a goodbye present from Sauron, in the shape of two buckets of fluorescent, slimy green goo. No matter how many times they magicked it away or tried to wash it off, it kept coming back… until Molly had enough of the twins tracking slime through the house, and went to St. Mungo’s with them to have it removed. All in all it was a rather harmless prank, Ginny thought. Knowing Sauron, he could have done a lot worse. Burning them alive or setting a giant carnivorous spider on them or something like that. The three weeks of itchy purple full-body rash they got from the counter-curse was only a minor inconvenience compared to what could have happened…
Much, much later, when she and Harry had long gotten together, and a shiny engagement ring adorned her finger, she discovered that he too had received a note from Sauron. Written in that recognizable elegant scrawl it stated,

To the Slayer of the Basilisk,

Ginny has six brothers to beat you up if you hurt her, so I shouldn’t mingle in this. Despite that, I must inform you that if you cherish having all your extremities, you better treat her well. She is in love with you, and if I find out –and I will, trust me- that you broke her heart, I won’t stop at just one hand to teach you a lesson.

Sincerely,
Sauron Gorthaur

Harry had admitted that the note had scared him so much that he had expressed his interest in her right the next day. Looking back on their relationship, Ginny decided that note was probably the best goodbye gift Sauron could ever have given her...

Meanwhile in Mordor...

The journey to Mordor was long and arduous, the daylong riding trips were hard on Sinistra, and they had to make-do with little provisions… but not once the wizard complained. She seemed both fascinated and baffled by her surroundings, a feeling Sauron knew well enough from his own experiences in her world. They didn’t speak much while riding, not knowing how to bring things under words exactly. It was an overwhelming experience for both of them, albeit for different reasons... For Sauron every inch of land held memories, often bad ones, while for Sinistra it was the complete unknown of her surroundings that made things difficult. He often wondered if she regretted her choice to follow him. He hadn’t lied when he said that life would be hard on her in his world… One night, as he saw her watch the stars with a melancholic look in her tired eyes, he sighed,
“It becomes less hard over time. I hope you… I hope you don’t regret your decision, and that one day you may call this world your home as well.”
With a smile she said,
“This world is already more my home than England ever was.”
And no matter how skeptical Sauron was concerning that, it turned out to be true. They reached the abandoned city of Minas Morgul -not completely poisoned and cursed anymore but still uninhabited- and made it their home, and Sinistra seemed to feel perfectly at ease there.
As time passed, plans and contacts were made, old alliances were called in, and the rebuilding of Mordor’s structures began, a little accident caused them to start building on something else as well, namely a family. The ground trembled and it stormed for days over Minas Morgul when Sinistra realized her condition and told Sauron… but eventually there was nothing to do about it, and a year later they welcomed Umátimon into the world, a healthy baby boy. It must be that having children wasn’t quite as bad as Sauron had expected, because approximately five years later they “accidentally” extended their family with another child, Mairë. Life in Mordor certainly wasn’t easy, with difficult diplomatic issues, language barriers, unruly orcs, the desert climate wreaking havoc on the harvest, the water purification engine getting clogged with ash every three days, and servants who just couldn’t understand that no, the flushable latrine wasn’t magical and no, you couldn’t use it to discard anything other than excrements. In general though, it was a good life, and Sinistra never came to regret her choice. She even discovered she was a rather good diplomat, although that was mostly thanks to the apparent fear people had of beings capable of magic.
In the meantime, Sauron decided that if he couldn’t take Middle-Earth by force, he would take it by means of Industrial Revolution… Sinistra watched with slight amusement how he implemented all sorts of technological improvements in Minas Morgul, built and rebuilt “modern” structures, made plans to have a train track running from Cirith Ungol to Barad-dûr, and generally filled his days with doing what he loved; planning, strategizing, commanding underlings, and working in his forge. Although he still made countless elaborate plans for attacking Gondor and achieving world domination, he somehow just never got to executing them. There was always something more urgent to be done, somehow… Maybe in a couple hundred years, when the children weren’t so small anymore, he’d get back to it…

(Author's Apologies)

So... yes, that's a lot happening in a little text. It's a bit fluffy... (but trust me you'll need it to make it through the next chapter...)
Basically Sauron and Sinistra went through the Mirror of Erised to Middle Earth. They didn't exactly ended up in Mordor though -interdimensional mirror travel isn't known for being very location accurate- so there was a long trip first. I suspect they didn't exactly end up in Gondor, more likely in Khand or Rhûn.
As for Mordor being homely to Sinistra: she comes from a desert climate. If anything the hot days and icy nights will remind her of her first home. Minas Morgul isn't really dangerous anymore, but it is basically a place full of bad memories and bad climate for just about everyone so that's why it's still empty.
And then Sauron who reproduces... Yeah, accidents happen. Especially when red magical ropes are involved and people use them for playing rather than marriage rituals. Umátimon means "Unconquerable" in Quenya. Mairë is the female form of Mairon and means "Admirable".

Little funny titbit:
Sauron fell through The Burrow (the Weasley house), and "burrow" is a synonym for a rabbit hole. Which basically made him fall through the rabbit hole into another world. And he leaves said world through the looking glass. Alice in Wonderland, anyone?