Setting a new bar for horror video games, Alien: Isolation has been terrifying players since 2014 with a masterful mix of oppressive atmosphere, dynamic frights and potent enemies. None are more terrifying than the titular Xenomorph — deadly, clever, and always seeming on the player's trail. With Isolation, Creative Assembly set out to recreate the horror of the original film, and succeeded.
But a big part of the Xenomorph's dread isn't due to level design, creature art, or even game mechanics: it's the Artificial Intelligence which drives it. In addition to being scary as hell, the Xenomorph in Alien: Isolation is also a landmark in dynamic video game AI. The team at Creative Assembly made an enemy like no other, which is more complicated and flexible than you might think.
7 It's Actually Two Entities
Taking inspiration from the flexible AI of games such as Left 4 Dead, the Alien's terrifying dynamic intelligence is actually achieved by two systems working together: the Director AI, and the Behavior Tree. The Director AI is omniscient, and can tell exactly where the player is and what they're doing at all times, as well as the location of the Xenomorph. However, it never tells the Xenomorph specifics or what to do.
That's where the Behavior Tree comes in. The Alien has a vast flowchart of possible responses to advice from the Director AI. Whilst it'll never be told details, it uses that information to guide its behavior: whether it's going to lay an ambush, actively enter stalking mode or just wander aimlessly around the area until the player messes up.
6 It Walks Just Like You Do
It's common in horror video games for enemies to teleport whilst offscreen, allowing them to attack players from unexpected directions at the worst time possible. The Xenomorph is so good at this that many players believe it's teleporting around the level too. However, this is false. It's just very, very good at slithering around.
The truth is, much like the player, the Xenomorph is limited to physically moving through the station during gameplay. This is a result of the relationships between its AI components, as the Xenomorph itself has limited information as to the player's location. The Xeno only teleports twice during the game's runtime, and these only occur during cutscenes so it can be in the right place for maximum terror.
5 It Has Normal Senses...Mostly
Because the Alien itself isn't all knowing like the Director AI, it has to use a set of sensory inputs to help inform what it's going to do next. Alien: Isolation players will be horribly familiar with the monster's keen sense of hearing, as well as its sense of smell. It can even use sight to change its tactics if it notices the player is armed. And, much like with its movement, the Xenomorph also has a limited, realistic range for all its senses.
Mostly. There is a secret sense that the Xenomorph has access to: motion sight. This essentially equates to it have weak eyes in the back of its head, and is used to prevent skilled players being able to sneak right behind its back. After all, this is a horror game - things can't be too equal.
4 It Literally Runs On Menace
One of the primary values for the Director AI is the "menace gauge". Menace isn't how many times the Alien kills you or shows up, however - it's meant to measure how tense the player feels at all times. By dynamically measuring different details such as how quickly the Alien can reach the player or how long it has been close by, the Director fills or drains the menace gauge.
It's not just constantly ramping tension, either. Creative Assembly knew that unrelenting high-level dread would become tiresome or even frustrating for players, so the Director AI keeps it at a constant medium. Once the menace gauge reaches a certain point, the Director will prompt the Xenomorph to give the player space and stalk elsewhere if they've not had contact.
3 It Learns From Pain
One of the few comforts in Alien: Isolation is the warm glow of the flamethrower. Although this weapon has limited ammo, it's one of the few items in the game that can temporarily drive off the Xenomorph if the player is in a pinch. However, just like a gamer learning to beat a hard level, the Xenomorph won't always be as afraid of the fire.
Over time, the AI will adjust the Xenomorph's perceptions of the flamethrower in accordance with how many times it's experienced it. Not only will the Xeno change its attack tactics to deal with the problem, it'll actually start hanging around for longer whilst getting blasted as it gets used to the pain. This can become a real problem, as eventually it can stay long enough to take a fatal swipe at the player before retreating.
2 It's Curious
Because the Alien isn't all-knowing, it spends a lot of its time hunting around the level for the player. There are certain places where it is flagged to simply stand and wait, but for many others it will just walk from room to room, occasionally even double-checking it hasn't missed anything. There's different behavior systems for when it's actively stalking the player, or just sweeping the level aimlessly.
Anyone who watches the Alien whilst it hunts around will notice that it also interacts with its environment, checking certain areas or objects. The AI has interest flags based on what it knows of the players position, and will think on its feet when searching to try and find a meal. It won't just go from Point A to B - it stops to smell the carrion on the way.
1 It Still Surprises Its Own Creators
The Xenomorph differs from most horror antagonists in that its behavior is almost entirely unscripted and highly intelligent. This means that rather than being able to plan for when it's going to show up, players must constantly react to it randomly appearing when they least expect it. As it turns out, that also includes the team at Creative Assembly.
The developers referred to playtesting against the Alien as "going on safari", because they could never predict exactly what was going to happen during each test. Sometimes they'd barely encounter the Xenomorph at all, and other times they would die within five minutes. It's a testament to their hard work in creating a dynamic and utterly terrifying enemy.