Somnia Kuran is Kaname Kuran's brother and the reincarnation of Sum of All the World's Evils. He is 2 years younger than Kaname. He is also a master of the 3rd True Magic: Heaven's Feel and has quite a bit of knowledge about the 2nd True Magic: Kaleidscope.
He was engaged to Kaname and was deeply dependent on him emotionally. It was how he was able to keep his abilities from corrupting him. However, at 5 years old the engagement had to be broken to engage Kaname to Yuuki for her protection. He didn't take it very well and broke down after their parents and Kaname left. His care for his younger sister made him hold his objection to it though he asked Kaname if he'll still love him to which he got to answer. He was able to repress the corruption through hoping that Kaname would still care for him but it keeps on dwindling every passing day that he doesn't visit him. It is also weakening him due to using all of his strength just to keep his ever-growing powers at bay.(mankind never stops committing atrocities~)
On his 17th or 16th birthday he's weakened to a point that a skilled human can easily defeat him. A Level E attacked him and was able to drain quite a bit of blood, weakening him further. Kaein Cross rescued him just before he lost conciousness. When he wakes up he's told what has been happening to his siblings. He's relieved to know that they are both fine and thanks Kaein for giving them a home and saving him. After Kaein left, Kaname came into the room and hugged him and berrated him for being reckless. They had a confrontation that ends up with Kaname letting him cry his heart out. After that Kaname left but not before hearing him say "too bad it's a day too late but thanks for letting me know you cared about me all that time anyway" and him giving a heartbroken smile.
After waking up, he's already nearly fully corrupted, having the Glyphs tatto of Avenger. He rushes to make a way for him to still affect his decisions even if under the corruption and a chance for him to return to his previous state later. After that he is fully taken over, his skin becomes darker and constantly emit a black and red mist. His true sense of self is then sealed and hidden deep within an abyss of shadows.
He then meddles with the Greater Grail for the Holy Grail War, modifying the it so that it can summon more than the original number of servants making it the bloodiest Grail War and making it the last one. He did this due to having the Grail Wars stopped being the last thought of the winning Master of the previous Grail War before succumbing to injuries. He does not care for good or evil, or, right or wrong, being desensitized from it due to his nature . He announces this to the world(mostly the ones that know of the supernatural), with Kaname and Kaein seeing it in person:
(does anyone mind making a grand poetic announcement about starting the last and greatest war for the holy grail... and maybe 'The Great Holy Grail War'?)
and ends the announcement with a lightshow that signals the start of the distribution of the command seals.
His entire visage and voice was change to those who didn't see him in person announce the start of the war.
at least 32 servants
He also states that every master must register after summoning their servants. Once everyone is accounted for, the fighting will then be allowed to start.
This basically has the same rules as the previous Holy Grail Wars.
He summons Enkidu and Gilgamesh both as Servant Ruler and inserting their souls into dolls he purchased from Touko Aozaki. He asks them to be the overseer for this Grail War and made them able to summon a Heroic Spirit each that can help them or hire people that can help. This is the influence of his true self as his shadow/corrupted version will simply not care about the destruction... kinda like how Avenger just accepts that some things happen whether good or bad.
At one point during his growing up(6-16) years, he absorbed the grand manifestation of humanity's grief and negative emotions to destroy it's sentience that is affecting him and Nyx negatively and also to help out Elizabeth due to him sensing her desperation when facing Erebus.
You can make it so that Angra Mainyu's counterpart will be absorbed by him later to save him or do something just make it so that his True self is given back and have a happy ending... or you can make it so that it will never end by making him(and sometimes a few other people) travel different realities/dimensions/universes.
P.S. I suggest making him go to Kamigami no Asobi! maybe as a teacher as a favor to Zeus...
yes... obviously yaoi/slash/shounen-ai
I got the idea from reading Shin Kuran by Icy Eyed Angel