Caster Limbo

"Ahh... Spetacular! It's amazing to watch as you regret, as you hate! This one feels so elated."

- Ashiya Doman in his teaser trailer of Cosmos in the Lostbelt

Caster Limbo, true name Ashiya Douman, is a major antagonist in Fate/Grand Order, serving as the main antagonist of Shimosa Province: The Stage of Rivers of Blood and Mountains of Corpses chapter in Epic of Remnant segment. He later appears as one of the secondary antagonists (alongside Kirei Kotomine and Koyanskaya) in Cosmos in the Lostbelt.

After being mentioned in the second and third Lostbelts, Douman first appears in the Cosmos in the Lostbelt as a supporting antagonist in its fourth Lostbelt chapter, Yuga Kshetra: Genesis Destruction Cycle. He is an Onmyouji in the Heian period and rival to Abe no Seimei.

He is voiced by Toshiyuki Morikawa, who also voiced Fiamma of the Right, Lord Boros, Griffith (in the 1997 anime version of Berserk) and Yoshikage Kira.

Shimosa Province: The Stage of Rivers of Blood and Mountains of Corpses
Under the alias of Caster Limbo, Douman is the instigator of an incident in a Subcategory Parallel World in Shimosa, before it was revealed that he is an Alter Ego-class Servant working for the Alien God and became an ally to the Crypters. His presence actually foreshadowed the presence of Lostbelts.

At the end of these events, Douman claimed to have finally succeeded in finding the Lost History (called the Missing Belt, which referred to the Lostbelts). He then mentions that the "Satan" he was following was just a false name, a ridiculous joke that he got onboard with a bit too far, before sincerely apologizing to a mysterious person.

Cosmos in the Lostbelt
Douman was mentioned in the second and third Lostbelt as an Alter Ego assisting Scandinavia Peperoncino. Unlike Koyanskaya and Kirei, Douman hasn't done much in the overall plot besides his scheme in Shimosa. It's explained in the prologue for the third Lostbelt that he's become incredibly fixated on something in the Indian Lostbelt and refuses to leave.

Trivia

 * He is the only main antagonist in Epic of the Remnants that wasn't working with/for the remaining Demon Pillars, neither did he was motivated with the result of their actions.