Take a ride
In order to hit 100% completion in Bloodstained, you need to gather up every item, shard, and locate every demon.
Here’s how to find Demon #44, which is one of the most obscure encounters in the entire game.
[Spoilers below with the appearance of the monster in image form and how to find it, with one location spoiler.]
You’ll be able to find this monster roughly at the midpoint of the game, once you unlock the train.
Once you’ve completed it, just head back to the main gate from the left side (the Garden teleport is one of the closest connections) and go to the end of the long hallway. Press up when the d-pad appears on-screen and select the board train option.
Once you’re on it, just head right one screen until you see a bunch of seats. Sit down and wait. Like, actually wait for over a minute. Once you start to see the castle more clearly in the background a demon will appear, in the same fashion as the one above, and invades your train car.
If you get close to it and look at it you’ll automatically be cursed, and he does an insane amount of melee-range damage, so it’s best to fight it from far away. The name of Demon #44, Kunekune, is derived from a real Japanese Slender Man-esque urban legend.