It's from an anime called the Fate Series. The first one made was Fate/Stay Night, which has two other alternate paths (same story, alternate universe basically) called /Heavens Feel and /Unlimited Blade Works.
Then there's /Protoype which was the guys original plan before his editor told him to switch the genders of the main characters, since manga was more male orientated when it came out and guys would enjoy reading about a guy who summon a powerful female warrior for stuff than a girl who summons a powerful male warrior.
There's /Zero, which is a prequel to /Stay Night and deal with the previous generation and expands on stuff that gets mentioned in the /Stay Night series. Then there's /Prisma Illya, which is another alternate universe where the ending of /Zero was different and it follows the younger sister of the main character from /Stay Night.
And that's all the anime I can think of off the top of my head.
The game we're talking about is a semi-separate story called /Grand Order, which employs a gatcha system for rolling for servants purely based on luck. Arthur and Hokusai are SSR servants, also known as 5 Star Servants, and have like a 0.5% chance to roll on a normal banner. When the servant is on 'rateup' the chance goes 1%. But even then you aren't guaranteed to get the servant you want, as once you roll that 1% chance, the game then internally rolls against every SSR servant in the banner to see which one you get.
Arthur is from Fate/Prototype, and is a limited servant in that he only appears in the Valentines banners. I don't know if Hokusai is limited. I rolled my first Arthur this year with my very last roll of the event - if I hadn't gotten him then I wouldn't have had a chance to get him until Valentines next year. And in rolling for Arthur, I got 'spooked' (what we call when someone who's not limited pops up unexpectedly) by Ozymandias himself, who was another one I wanted but not the servant I wanted at Valentines.
I'm extremely salty at my alt account though. Toss a ticket at whatever banner it currently on...and almost guaranteed to get the SSR on rateup. It has a 1:1.6 ratio for 5 star vs 4 star servants, while my main account has a 1:4 ratio. That means for every four 4 star servants, which is easier to get, my main account has 1 5 star servant. While my alt account gets a 5 star servant almost every third gold servant he summons.
I'm still salty as hell about him managing to roll Berzerker Vlad and Scheherezade in the same roll. Like, FFS.