This mod is Slenderstalgia (aka Slenderman mod) port/remake for 1.12.2, with tweaks and improvments.
He stands still and sometimes teleports to the player, casts negative effects, and attacks him when he is too close. He also has an attack that pulls the player towards him, preventing him from escaping.
Breaks light sources and doors.
Slenderman can appear at night on the surface with some chance near the player. The higher the difficulty of the world, the higher this chance is. Player can prevent the Slenderman from spawning by wearing a full set of Slender armor or by being near the respawn point (50 blocks by default).
The basic material for crafting the other items. Can be obtained by killing Slenderman.
Can also be eaten, it will strengthen the player but also cause negative effects on him
Poisons mobs on attack. Deals x1.5 damage to Slenderman
Makes it easier to fight Slenderman by preventing some negative effects and visual effects
By themselves don't do anything special, but a full set of this armor prevents Slenderman spawning
“Fast render” option breaks mod's visual effects
