Slenderman is a largely adapted internet creepypasta. In most fanon adaptations of creepypasta, he plays a role of a "father" for his proxies and other creepypastas in a setting commonly titled "The Slender Mansion." Canon Slenderman is known to lure children into the woods and murder them and psychologically torture other victims. For more information, here is a wiki page on Slenderman and his origins:

https://creepypasta.fandom.com/wiki/The_Slender_Man

This role will be needed only a few times throughout the comic.

Ideally, this voice should be deep and growling with a cold tone.