No, the horror video game series "Slendrina" is not based on a true story. The game's storyline and characters are entirely fictional and were created by the game developers at DVloper. The game series follows the story of a vengeful ghost named Slendrina, who haunts an old house and seeks to harm those who enter it.
The Slender Man (also spelled Slenderman) is a fictional supernatural character that originated as a creepypasta Internet meme created by Something Awful forum user Eric Knudsen (also known as "Victor Surge") in 2009.