but did you die?

The short answer is that Wendy is May and Slendy's kid, and Maw is May and Zalgo's kid. As to why that happen... that goes back a long way in blackbirch AU.

Zalgo/Shormash Urdal is a primordial Lovecraft-esque elder god in the au, and this guy eats worlds, literally. He seeks out civilisations, devours their guardian deity/ies, and then drains the life out of their habitat until the whole realm or planet crumbles into dust. Sometimes he takes a few of their people, generals, warriors, concubines for his 'court', bound to him by arcane silver chains. They are called the right and left hand court, respectively.

When his court inevitably rioted, they took out his right hand and the surviving members scatter to different worlds. Some came to ours, assimilating into human host bodies selected among Zalgo cults, headed by former generals and sycophants who remained loyal to the elder god. But some pick their own hosts randomly as they come across it, which is the case of one of his wives.

Back to the human world. May Rosewood is an orphan whose single mother wasted away, and she lived in foster care til her grandmother contacted, wanting to adopt the kid back into the family. The day she was supposed to leave, eight-year-old May ventured out in the woods and saw a ghostly woman whose shape shifts and ripples painfully with each move she took. The woman turned into a child to lure May out, then took over her body as she blacked out.

May woke up, shaken but otherwise unharmed, and was immediately took to meet her grandmother. The shock became a forgotten memory and she lived the rest of her childhood with relative inconsequence.

Moving back home at age 34, an adult May confronted the truth about her origins. She also discovered that she was carrying Zalgo's offspring, a plan orchestrated by Zalgo's cult and her fiancé. Betrayed, she furiously delved into elder mythos to find a way out, and that was when Slender came into the act.

She stole a summoning sigil, gathered what she needed and managed to call upon Slender, who took her under his protection as they look for a way to eliminate/mitigate Zalgo's influence on the fetus. Slender has been trying to create Slenderkin children/spawns of his own for a while; he does this by uhhh [GORE WARNING] vivisecting infants and injects his own blood into their spine [END GORE WARNING] Sometimes it works, sometimes it doesn't.

But waiting until May gave birth was risky, as some Zalgoid can be extremely powerful even as newborns. So he fed her his blood directly and kept her on close watch, intending to kill her if anything went remotely wrong. But it never happened, and they instead developed a sort of wacky relationship, unlikely friends slash romantic partners, right up until the baby was born.

this is their relationship dynamic. no i don't take constructive criticism.

And then you have Wendy, who apparently has no Zalgoid influence except for Maw who lives inside her head (for now). In other words, Maw was the original Zalgoid spawn that their mother carried, but eaten by her chimera twin Wendy at birth. Her Zalgoid powers are subdued but it's still there, hidden inside their shared body, waiting for a chance to come out.

As for if Slender knew this... I'd say he is aware there might be a chance his daughter could fall back under Zalgoid influence, but he couldn't have predicted they'd be two different people.

anyway Thank you OP for asking the vital question that made me actually sit down and realign my silly little plot points!!!!! this is me the entire time i wrote this:

also enjoy some good ass art from my amazing insta mutual Sandie!!would die maim kill for them