SPOILER ALERT: This story discusses major plot points, including the ending of “Alien Romulus,” now playing in theaters.

Fede Álvarez might have just delivered one of the wildest endings of the year with “Alien: Romulus” — thanks, in large part, to its terrifying new monster, nicknamed “The Offspring.”

Early in the film, Kay (Isabela Merced) tells Rain (Cailee Spaeny) that she’s pregnant. As the Xenomorphs unleash terror on Rain and the crew, Kay is captured by one. Rain and Andy (David Jonsson) find her in a cocoon and free her. As the trio fight to escape the alien-infested space station (which is on a crash course with a nearby planet), Kay injects herself with the Weyland-Yutani “Compound Z-01” – made from matter extracted from the Xenomorphs and designed to advance humanity. But when Rain puts Kay into a cyro-chamber so they can get her back safely, something goes horribly wrong and Kay begins to give birth — to something rather horrific, a baby that rapidly evolves to a human-xenomorph hybrid.

“The way it’s described, you’re trying to picture it in your head, and you don’t know how the director is going to create this character,” says visual effects supervisor Daniel Macarin. “Is it going to look like a Xenomorph? Is it going to be something very unique? Is it going to be something we’ve never seen before?”

As it turns out, it’s all of the above.

Because Álvarez wanted the film to be terrifying and favored practical effects over CGI, Macarin and animation supervisor Ludovic Chailloleau scaled back, helping to emphasize the work of the creature design team, but in a subtle way. Everything about The Offspring needed to be natural, with the less is more approach.

On set, The Offspring was played by Romanian basketball player Robert Bobroczkyi — all 7 feet 7 inches of him.

“The first time saw the plates, and this 7-foot [tall] actor in there, in his costume. It was terrifying,” Macarin says. “They did such an amazing job with the look and the feel of that character that we knew that everything that we could bring to it was just going to help.”

Because Bobroczkyi isn’t a professional actor, Chailloleau explains, “the way he moves is just unique,” which informed the way they animated the character. “Considering his big size and the concept of what [the creature] is, he’s giving a lot of new things to watch visually, so I found that to be outstanding.”

When Kay gives birth, the baby was a practical effect. “They put it in the egg, designed and dressed it,” Macarin says. Except the baby didn’t move — and Álvarez wanted audiences to understand that despite Kay’s gory labor, the baby was alive. Macarin’s first dilemma came in finding the right facial expression for the baby Offspring. It couldn’t look cute and it couldn’t be too angry. “You don’t want [the audience] being that empathetic with the baby, you still want them a bit terrified,” he explains.

Once the baby begins growing (at a rapid pace) and starts crawling towards Kay and Andy, Macarin added in tiny details such as extra blood dripping out of the holes in his back, adding movement to the back of its head and pushing the skin around a little bit, “just to give it that little extra bit of horror that the audience will cringe and react to,” he explains.

They also enhanced the creature’s tail, with Chailloleau cycling through numerous iterations of how it would move. Was it moving enough? Was it a part of the body or something with a different brain? They didn’t want it to look like a dog’s tail or a cat’s tail, Macarin notes; it couldn’t make The Offspring look too happy or excited.

“This is a homage to the ’80s, in design style,” Chailloleau says. “Even the models and the practical [effects], everything has to fit the way they would have done that years ago. We don’t want to look to CG, so motion is very important, because we could not animate that way 40 years ago.”

In the end, he looked to Bobroczkyi’s performance, and he took inspiration from Ridley Scott’s “Alien” and James Cameron’s “Aliens.”

“When the first ‘Alien” was done,” Chailloleau says. “It was someone in the back with string and with a bit of help to just put the tail up and the rest followed. … So, the tail is not the focus of the shot; it stays secondary, but it’s still very menacing.”

Adds Macarin: “You don’t get a lot of screen time with this character, so it’s making sure that the audience never has the wrong reaction to it. If they giggle at it or something comes off as funny, then you’ve taken them out of the moment, and we don’t have enough time to probably get them back into the action.”

The Offspring’s mini-mouth also went through several iterations during the design process. The Xenomorph mini-mouth was iconic, hard and straight with hammering teeth, while The Offspring’s juvenile version needed to be softer while edging on dangerous because it’s still growing.

In the scene, The Offspring moves towards its mother Kay, and the idea was that it was looking to feed. So as it lowers its head, Chailloleau had to make sure the mouth’s movement was connected to The Offspring and that it didn’t have a mind of its own. “It was making sure that people didn’t think that he was trying to attack her, but the mouth is still dangerous,” Macarin says.

Toward the end of the sequence, the action moves out into space (as the station begins to collide with the planet’s rings), which presented its own set of challenges. In terms of The Offspring, there was only so much that Bobroczkyi could perform.

“When they start saying, ‘We need him to hang from the ceiling. We need him to drop down,’ the actor can’t possibly do that – not in any safe way, anyway,” Macarin explains. “And we can’t replace them with a stunty, because we don’t have 7-foot 2 or 7-foot-5 stunties of his proportions that can do these either … So anything out in space, [Chailloleau] had to take over and build a fresh performance, drawing back on everything that the actor had done in earlier shots.”

Audiences have left the theater wondering about The Offspring’s close resemblance to the Engineers in “Prometheus” and Macarin affirms that referencing the larger “Alien”-universe was the filmmakers’ grand plan.

“It’s not something that you want to directly connect,” Macarin explains. “But if you hint that there’s a larger story there, there’s more mystery, and maybe we’re just seeing the beginnings of those ideas, it was definitely something that we wanted to explore.”