i'm so glad someone asked this because YES. I DO HAVE THEM!
before i preface all of my headcanons, i should explain briefly my interp of them both
starting with slenderman and the million other names he goes by (zalgo calls him by all, the operator, the administrator, the tall man, gorr'rylaehotep .etc .etc)
i envisioned him to be an entity that takes and takes with an affinity for nurturing potentials and basically giving them the push (the sickness) to flourish. he does not have the iconic slendermansion because he exists everywhere, no matter the place or time. the concept of being in a relationship is very foreign to him because he does not have a need for it but.. he has favourites. always has.
like it was with the EMH crew, alex kralie and by extension the MH crew and all the people in the slenderverse series. he's familiar with the rake and the HABIT. but zalgo?
zalgo was never planned to be one of them.
zalgo is a lower-caste demon, he's unlike the "god of hell" interp that people usually have. he's very in tuned with the current era as his influence spreads through "zalgofied pictures" similiar to that of smile.jpeg. his power is as strong as people's memories of him, that's how reliant he is on humankind despite despising them with all of his blackened heart. he observes humans and their nature, mimics them to an uncanny degree and walks through earth rarely noticed for his true being.
unlike slenderman, demons are creatures that doesn't necessarily takes by force, but it lulls you into consent. through contracts, seals and rituals. this is how zalgo operates.
and when you're dealing with the same type of prey, you're bound to stumble into each other, one way or another.
slenderman is territorial with his catch, he's never one to share. he oversees the overworld, so when one of his catches has made a pact with a devil. they're protected by hell's wrath and there's no way for him to get his hands on them anymore. he was furious, and through the fit of rage that the entity displayed, zalgo finally decided to show himself - his true self, to another entity of the overworld.
yes, it made him vulnerable because it made him gained a physical form.
yes, it pleases slenderman when he could finally have a target for his anger.
but zalgo only taunted him more. a smile. dozens - no hundreds of them. it appeared on his body, the souls he's acquired, grinning without end at the tall faceless man as he was stabbed by the tendrils. zalgo could see into the interdimensional shift of all the possibilities as he was impaled by each tendril and that gave him an idea. a horrible one. finally, the demons have found the mountains of avenues to spread his influence.
he retreated safely, his body fusing back down into the earth yet not a speck of himself was found beneath the dirt. back to hell, back to home. slender couldn't get him.
and that infuriated him more.
and that's how their first meeting goes. the rest would be a history of tug and pull. slender never saw him as an equal yet zalgo did. zalgo was always the one more in tuned with humanity anyway and that caused him to seek the thrill more, the danger and the thought that he could perish. not under an angel but something else, a rot that he would subconsciously cling to for the coming millennia.
i envisioned them to be unfathomable in size, appearing as whatever the humans wanted to see them as. but to each other, their bodies would be smaller than the tallest of mountains and bigger than the tallest of hills. when in this form, they tower almost everything in sight. usually, it's not possible to see with a naked human eye but for the people who are sick, contracted, diseased, their large physical bodies will be visible even in the dead of night.
and that's all lol. i realized this went onto yapping territory but i got a lot to say!! ive never had anyone asked this before so im glad i can finally let this out. theyre kinda my guilty pleasure cause no one in my CRP circle really gaf about them LMAO