I'm not sure exactly how old I was when I first read What If? v2 #30; I don't believe I bought it new, so I must have been older than nine, but I also don't think it was long after it came out, so I'll ballpark myself around eleven.
As you can see from the cover, it was "two feature length thrillers" asking the question "What if the Fantastic Four's second child had lived?" Comic Vine has Jim Valentino listed as the book's writer and (ironically) current FF artist Dale Eaglesham as penciler, but I'm not sure if they did both stories or, if not, which one they were responsible for. I'm not concerned with the second story for the purposes of this post, as it was a fairly pedestrian yarn about Reed and Sue Richards' daughter becoming some sort of global messiah, but the book's first "thriller" scared the living heck out of me.
The premise of the story, as implied by the question/title, was presenting one possible scenario had the second child of Mister Fantastic and the Invisible Woman lived. See, while today any FF fan worth their salt knows that Reed and Sue do have two children, Franklin and Valeria, back in 1991, Franklin was the only child of Marvel's First Family, as a second had been lost in childbirth during John Byrne's run on the book due to the mother's exposure to radiation in the Negative Zone (I believe).
Here, with the help of Doctor Octopus, Reed is able to save the baby, a girl, but Sue dies in the process.
The story is told from Franklin's point of view, and right off the bat Valentino (or whoever wrote this) provides an emotionally gut-wrenching account of a little boy losing his mother and having to accept that the person who ostensibly killed her is now a member of his family. That's not even the makings of a horror movie, that's a real-life tragedy that was certainly not typically dealt with in all-ages super hero comics during the 90's, not something I had ever really thought about at my tender age, and a subject I at least remember the creative team handling with impressive care and craft.
However from there creepier elements beyond the norm do begin cropping up as members of the Fantastic Four as well as friends like Alicia Masters are becoming weak and dying and nobody can figure out an explanation why. Franklin is the only one convinced his younger sister has something to do with it, but his father refuses to believe him and his uncles and their super friends humor him but try to explain to him it's only natural to feel threatened by a new sibling (which it is).
There's really no on-panel action or violence for the first two thirds of the story, just personal dynamic drama about your family being thrown into tumult and nobody believing you; it felt more like "Child's Play" or any other thriller designed to terrify kids specifically because it focuses in on your helplessness to save the people you love because you can't get anybody to listen to you.
Again, I can't say for sure that Dale Eaglesham was the artist on this story, but if he was, even at what would have been a formative stage in his career, he was doing some impressive work, as his visuals were a huge part of what made this story so dang eerie. I vividly recall a panel of Franklin walking by the room of an emaciated Johnny Storm who was half-conscious and mumbling as one of the scariest images my young mind had ever processed. This was the Human Torch, a cocky, young character full of life sapped of all that and there wasn't even a bad guy to throw blame to; gripping stuff when you're eleven (or twenty-seven).
Reed basically locks himself in his lab and becomes obsessed with figuring out what had happened to his family at the expense of paying any attention his son, who insisted his sister was to blame, a claim Mister Fantastic refused to hear. This worked because it was a pretty logical extension of Reed's character (how many times has he locked himself in a lab and ignored his family over the years?) and made the next bit even more shocking but weighty when he hauled off and slapped Franklin for daring to hurl these accusations.
Obviously this story played on some pretty realistic fears most kids have: your younger sibling trying to steal your family from you (literally in this case), nobody listening to you, and even a parent who is supposed to be protecting you turning against you in the cruelest of ways. This wasn't alien invaders or monsters from another dimension, it was real stuff that some kids are unfortunate enough to face jacked up with sci fi trappings.
The climax of the story is almost predictable, but no less horrifying, as the youngest Richards does indeed turn out to be a parasitic Negative Zone monster with emotion-altering abilities who turns into a giant green creature after Franklin recruits Doctor Doom of all people as his savior while his father curses at him to his last breath. The beast ends up tearing through both Reed and Doom and trying to come at Franklin, who is able to activate a switch and send her back to where she came from. The story ends with Franklin cradling his dead father's head and wishing somebody would have listened to him.
It was a horror movie in comic book form done to great effect and pulled off with maximum psychological effect. It terrified me so much I gave the comic to my mom and had her hide it from me, never wanting to see it again. I believe when I was in college we were cleaning some area of the house and I found it; I read it again, all my childhood emotions came rushing back, and I threw it out post haste.
It was the scariest comic I ever read.