(This is for the fanfiction "Crossover Together". Info may change from canon information)
Sombra is the a wolf-class Umbra Demon, main antagonist of Crossover Together, and the current ruler of Tartarus. He was once a commander from Elysium until his encounter with humans drove him back to the Underworld.
He is described as being as tall as Sebastian Michaelis. He has white, ghostly pale skin with long black hair that reached his shoulders, extending with a set of sideburns and a small goatee beard. His eyes are reddish hot pink, with a satanic star for irises, with a red diamond at the center of his forehead.
He wears a silver crown with crimson crescents and a black pearl at the center, and his outfit is described as matching political leadership: a grey overcoat, black pants and black inner shirt, the shirt having red borderlines along the collar and bottom rim. Along his neck, he has a pure white tie, splitting into two points at the end, and wears a set of jet black boots.
Sombra is a open-minded, calculative, and manipulative demon who is more prone to using word and mind power rather than physical strength to further achieve his goals and motives. He usually thinks forward in the future, and much of his decisions are planned with that in mind, accompanied by his Future Sight ability. He has an ill feeling towards humanity, and doesn't show much compassion towards them apart from a selected few, but only if there was something to gain from it. A notable example of this was his willingness to help Gaara, only for the sake of releasing Shukaku from him and as a possible advantage. His main goal was to wipe humans off of the Surface World, though it is revealed that he was testing them to see if they can truly adapt to their worlds connecting.
He doesn't consider Homunculi worth caring about, as not only are they low in ranking, but he considers them false humans more than most, yet he still cared enough to keep them around, shown in his refusal to kill them by Chrysalis's suggestion, and that he still allowed Envy time to heal after he was almost burned to death by the Phantomhive Manor servants. His true compassion is towards his fellow demons, and he tried to make decisions that result in the least amount of causalities.
Although a demon, he does hold a strong respect to the rules, and will keep by the rules he placed along with the rules placed before him to avoid chaos. This is something he enforced onto his followers, and those who continue to cause trouble or break said rules he would personally punish. He refused to involve himself with especially insane individuals such as Tirek and the Great Tengu.
While it was unclear as to how he managed, considered impossible prior to him, Sombra had raised enough ranks to be placed as an Elysium commander under Celestia. During his patrols, he began to take some note of the pollution the Humans were causing. His worries became even more prominent when one of his angels flew back with a wing shot by one of them. He reported this to Celestia, who told him to just stay out of the way as it was the best they can do. The following night he was visited by Undertaker, checking in on him and after he foresaw Pluton's release by Tirek, suggesting he'd try and handle the problem himself by returning to his demonic roots. He tried once more to get Elysium to act, and when refused, he removed his status himself and left Elysium, and therefore the Overworld altogether.
He returned to the Underworld, and at a point he confronted Tirek. He challenged him for the throne, and managed to win, but instead of killing him he had him isolated and locked away. He became notable ruler to Tartarus.
Sombra, while not initially appearing physically, had appeared through his communication with Envy, insisting that he stay well out of sight. He himself possessed Gakuho Asano later on during the final exams of Orora High School, calling Pinkemina Diana Pie, Twilight Kagayaki, Karma Akabane, and Nagisa Shiota to the office. There he explained that he was going to start a "game" with them all, and that they simply have to keep up with him, hinting of the apocalyptic outcome at the end of it before he disappeared.
His first official appearance was when he met up with Lust and Envy in England, where he met Sebastian Michaelis. He soon saw his Faustian Contract, seeing how he was so hesitant during the initial offer the first time around, and when placing a tracking spell on him explained that he will remove it once he'll get settled back home. On his return during one of his meditations, he was contacted by Tirek, who began to question him and why he was holding himself back, much to his anger.
In another meditation, he was visited by Sebastian, who offered him a drink, to which Sombra insisted that he didn't have to do that anymore. He began to question him however, namely on his lie that he killed Ciel Phantomhive, Sebastian telling him he didn't want his "meal" taken from him. Sombra promised him that he'll find a way to get the soul for him at a later date.
As everyone was recovering, Sombra went to check in on Envy, Lust, and Gluttony, before he went into Tirek's cell to check on him. Tirek mocked him, saying that he was only the "match to start his hellfire". Sombra scoffed it off as he was locked up and left him there.
He grew upset when Lust told him of Envy's capture in Amestris, but it quickly turned to panic when he discovered that Tirek escaped. He quickly had the palace on lockdown and that none of them are to leave until he came back, knowing where tirek was going already. When he caught up to Tirek, he tried talking him out of releasing Pluton, to the point of pleading, but to no avail. He tried commanding Pluton to stop, but he was powerless to stop him and Sombra could only leave, hoping that it wouldn't end everything.
After the encounter, he returned to find Envy (who was human), and believed that he was responsible after some of his hair was found by the cell. He ended up disproving it after a while, but he still punished him by forcefully removing the Philosopher's Stone from him, claiming he was going to take away his powers anyway but Chrysalis had done it earlier than expected. He made it clear that Envy was now a human being, and he left him to his punishment.
As suggested by Chrysalis, he decided to go for the humans for some aid in his approach. He began with Gaara, who he only managed to convince after saying he'll remove Shukaku from him. With him, he went over to Amestris, and got both King Bradley and Selim Bradley (Pride and Wrath) to help him as well.
Afterwards, he was confronted by Gaara, who was getting impatient with him, and he got to trying to remove Shukaku from him. He found that not only was Shukaku as bad as Tirek, but was planning to leave himself in the near future anyway. This had sombra leave and said that he couldn't do it yet, much to Gaara's anger.
Near the end of the season, he got Selim to get Envy for him, which he planned to burn alive in lava as a way to "help" him. Sombra though quickly realized that he got Nagisa Shiota instead, and after some talk decided to finish him off instead, only to be stopped by Edward Elric and Alphonse Elric.
On the Campania, Sombra encountered Sebastian Michaelis, and explained to him that he fulfilled his prior promise to him, presenting a device that would suck out a human's soul for him. He was disappointed when Sebastian explained that he would die if he did so without completing the contract, but was given a well-enough plan to have Sombra agree to. He appeared later on, and after Sebastian died, he took Ciel Phantomhive away back into Abaddon, locking him up there himself.
He went off to try and find Shukaku, but found he was too late and that Shukaku was already dead. Regardless, he caught up with the group, and used Hoopa's rings to teleport Aladdin to the island where Lugia was residing. He used that to finally find him, and readied to possess him before Morgiana intervened, and made Sombra flee.
After getting most of Team Harmony, Sombra had them sent off to Abaddon with Ciel on request to the Slicer Brothers. He didn't realize that they were a few short though until later, much to his stress and he nearly killed the Slicer Brothers on the spot for such. He decided to use them as bait for him, his future sight telling him they would show up. He didn't plan however on his friends showing up, and Envy had tried to kill him on revenge. He made it clear that it was his own fault for all his reckless behavior, and that he truly did try to return him as a Homunculus for his former friends' sake. He soon found later on that the Great Tengu had escaped, and he had to fight him. He reappeared later on, giving a slight applause to them as they left.
Later on he grew especially stressed when he found that Gaara took all his plans and documents, leaving him with little planning to defend himself with. Stressed and worried, he was almost ready to give up when Wrath, Pride, and Chrysalis reassure him that they will win. Calming down, Sombra left the palace and went to Elysium one more time. He found Team Harmony and gave them all an offer for one final confrontation, and that whoever wins will decide the fate of the Human World. Upon agreement, he wished them good luck before returning.
In the final fight, Sombra was informed that the Slicer Brothers, Lust, and Gluttony had left him, and he went on to send Pride to the Homunculi chamber as a mast minute line of defense. He worked out a portal to the Surface World, and just managed to slip away before he could be killed by the assassination squad. On his arrival, he appeared in the forest outside of the E building, and he found what he thought was Nagisa up in a nearby tree, only to find that it was Niji. He was forced to run away, getting caught in numerous traps set by the humans and even getting shot by Ryoma Terasaka. He managed to trick them in a last-ditch effort, and finally found Lugia with Ciel at the E building. After a conversation with Ciel, he went to possess Lugia, believing to have won their game and went to destroy Tokyo. During this, he was confronted by Nagisa, being told that Humans shouldn't be given this punishment. Sombra stated that he didn't believe any of them would change, but was told that he himself was a example that anyone can change. Angry, he used Lugia's Aeroblast on him, just to find that he had his powers sucked away completely, alongside Lugia. On his defeat, he admitted that they won the game and that they were capable of standing up to his world and theirs, willing to take what punishment they had. As he said theirs however revealed to everyone that he was never planning to kill them all, but to prepare them, and that this truly was all just some game and nothing more. He stated that he knew all of this would happen and that he wanted them all ready for it.
In the end, they all allowed him to live, stating that he had his kingdom to look after, and that he had more morals than most other demons down there, them even thanking him for everything. Humbled, Sombra left them all so he could clean up everything, silently stating that he only one that wasn't ready was him.
On their first encounter, Sombra didn't see Nagisa as any better than the other human beings. As time went by though he began to become more and more aware of his involvement, finding him very interesting and considered him a intriguing target. He began to show more respect towards him after his fight with Tirek, and when confronting him directly, actually gave him advice for their upcoming final fight and was comfortable enough to tell him his full motives and reasoning.
It's a little unclear as to his exact relationship with Lugia. He seemingly had little respect after his days as an Elysium commander, as his final act were to possess him to destroy the world.
He personally never liked Tirek in the slightest, and while he still had some sympathy towards the demon king to let him live after taking the throne, he kept him locked up underneath Tartarus. He grew especially angry with him after Pluton escaped because of him, and actually gave his gratitude and thanks to Team Harmony for killing Tirek for him.
Chrysalis is a newer member of Sombra's kingdom. At first, Sombra mainly brushed off much of Chrysalis's attempt to flirt with him, though overtime he began to slowly warm up to her.
Envy was one of Sombra's more notable followers, and Sombra usually tried to keep him as a spy. He valued Envy enough to keep him around thanks to his abilities, but as Envy became more and more reckless, it got to the point when Sombra would send Chrysalis to drain his abilities and he himself removed Envy's Philosopher Stone, making Envy fully human and of no more use to him.
When she flew back to Elysium after getting her wing shot, Sombra was especially concerned over her safety, and he himself got the bullet removed from her wing. Undertaker notes that it could be because of a possible crush on her.