Fate/Grand Order - First Order
Yes, they made a single-episode anime special out of this. No, don't stop reading yet.
This ended up being way, way better than it had any right to be. It actually was pretty damn good. As you might guess from the title, it covers the story of "First Order", the Prologue episode from the game. The actual game became less and less serious as time went on in terms of story, but the Prologue plays the premise completely straight and so does this anime.
The actual animation is extremely decent and the fights are convincingly Fate. The characters are represented very well and gain additional backstory and motivations over what little is seen in the game. It's actually enjoyable enough that it would have worked out as a full-length series if the story of Grand Order actually had stayed more or less on point.
They were able to fill 72 minutes up with this and it didn't drag at all. As animes based on mobile gacha games go, it measures up to the strangely high standard set by Rage of Bahamut: Genesis.