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.

“With the way it's described, you try to visualize it, and you're unsure how the director will bring this character to life,” explains visual effects supervisor Daniel Macarin. “Will it resemble a Xenomorph? Will it be something entirely original? Will it be unlike anything we've ever seen before?”

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

Because Álvarez desired the film to be terrifying and favored practical effects over CGI, Macarin and animation supervisor Ludovic Chailloleau scaled back, highlighting the work of the creature design team, but subtly. Everything about The Offspring needed to appear natural, with a minimalist approach.

On set, The Offspring was portrayed by Romanian basketball player Robert Bobroczkyi — a towering 7 feet 7 inches tall.

“When I first saw the set, with this 7-foot [tall] actor in his costume. It was frightening,” Macarin says. “They did an incredible job with the look and feel of that character that we knew that whatever we could add to it was only going to enhance it.”

Since Bobroczkyi isn’t a professional actor, Chailloleau explains, “the way he moves is simply unique,” which influenced how they animated the character. “Given his height and the concept of what [the creature] is, he’s providing a lot of visually captivating things to watch, so I found that to be remarkable.”

When Kay gives birth, the baby was a physical effect. “They placed it in the egg, designed and dressed it,” Macarin says. However, the baby didn't move — and Álvarez wanted viewers to understand that despite Kay’s gruesome labor, the baby was alive. Macarin’s first challenge was finding the right facial expression for the baby Offspring. It couldn't appear cute, and it couldn't be overly angry. “You don't want [the audience] feeling too much empathy for the baby, you still want them to be a bit terrified,” he explains.

Once the baby starts growing (at an accelerated rate) and begins crawling toward Kay and Andy, Macarin added in subtle details such as extra blood dripping from the holes in its back, adding movement to the back of its head and slightly adjusting the skin, “just to provide that little extra touch of horror that the audience will recoil and react to,” he explains.

They also enhanced the creature’s tail, with Chailloleau exploring various possibilities for its movement. Was it moving sufficiently? Was it an integral part of the body or something with a different mind? They didn't want it to resemble a dog's tail or a cat's tail, Macarin notes; it couldn't make The Offspring appear too joyful or excited.

“This is a tribute to the ’80s, in terms of design style,” Chailloleau states. “Even the models and the practical [effects], everything has to match how they would have done it years ago. We don’t want to rely on CGI, so motion is very important, because we couldn’t animate that way 40 years ago.”

Ultimately, he looked to Bobroczkyi’s performance and drew inspiration from Ridley Scott’s “Alien” and James Cameron’s “Aliens.”

“When the original ‘Alien’ was finished,” Chailloleau explains.  “It was someone in the background using string and some assistance to simply raise the tail, and the rest followed. … So, the tail isn't the primary focus of the shot; it remains secondary, yet it’s still quite intimidating.”

Macarin adds: “Because this character isn’t on screen for a long time, it’s crucial that the audience always understands their role. If they laugh at it, or something seems humorous, it takes them out of the story, and we wouldn’t have enough time to get them back into the action.”

The Offspring's miniature mouth also went through several iterations during the design process. The Xenomorph mini-mouth was iconic, rigid and straight with sharp teeth, while The Offspring's juvenile version needed to be more flexible while still hinting at danger because it's still developing.

In the scene, The Offspring moves towards its mother Kay, and the intention was that it was seeking nourishment. So as it lowers its head, Chailloleau had to make sure the mouth's movement was synchronized with The Offspring and that it didn't appear to have a mind of its own. “It was about ensuring that people didn't perceive it as trying to attack her, but the mouth is still a threat,” Macarin says.

Toward the end of the sequence, the action shifts to space (as the station begins to collide with the planet's rings), which presented its own challenges. In terms of The Offspring, there was only a limited amount that Bobroczkyi could physically perform.

“When they start saying, ‘We need him to hang from the ceiling. We need him to drop down,’ the actor simply can't do that – not in a safe way, at least,” Macarin explains. “And we can't replace them with a stunt person, because we don't have 7-foot 2 or 7-foot-5 stunt performers of his proportions that can do these either … So anything taking place in space, [Chailloleau] had to take over and create a fresh performance, drawing on everything the actor had done in earlier shots.”

Audiences have left the cinema pondering The Offspring's striking similarity to the Engineers in “Prometheus”, and Macarin confirms that referencing the expansive “Alien” universe was the filmmakers' overarching plan.

“We didn't want to make a direct connection,” Macarin explains. “But if you suggest there's a broader narrative, more mystery, and perhaps we're just witnessing the beginning of those ideas, that was definitely something we aimed to explore.”