Entity Profile: NPC_Zombie

Note the lack of a Headcrab – these only appear in-game.

The standard Half-Life 2 Zombie is one of the most basic humanoid NPCs in the game, but there’s still quite a deep level of complexity build into how the NPC operates, reacts to its surroundings and the way it can interact with environmental objects.

This entity profile will be an in-depth look at the Zombie, it’s traits as both an NPC and entity, and how it can be adjusted or controlled by mappers.

The Zombie As An NPC

The regular Zombie is very similar to the kinds of zombies you’ll see in horror films – slow, durable and not very bright, with no way to fight at a distance and very limited coordination. However, unlike your average “horror movie zombie”, Half-Life 2’s undead are still fairly smart when it comes to navigation, and this can be improved on even further with certain mapping tricks or techniques. Zombies are some of the most common enemies in a lot of Source Engine mods, mainly because they’re easy to work with – you don’t need to create complex scripted sequences of them talking or using equipment to make them believable, threatening and sometimes downright scary.

Zombies have different behaviour to gun-toting enemies. Their primary goal is to get to the player and attack them, regardless of the situation, so they’ll stop at almost nothing to close the distance even if it puts them in harm’s way. A good example of this is the spinning trap in the base game’s Ravenholm section – whereas other NPCs will generally avoid a moving object that could kill them, a Zombie will wander right in and get sliced in half, since they’re focused on nothing but the player.

To facilitate this kind of behaviour, they have an ability that no other class of NPC gets: they can shove objects out of their way or towards a target they’re chasing, causing minor to significant damage to the entity it hits depending on what the thrown object actually is. This can also break down wooden barricades or cause fragile items to shatter or break. This isn’t really a ranged attack, but it can still act like one if they’re lucky enough to hit the player.

There are two additional events that can happen when a Zombie is killed, neither of which are directly controllable by the player. The first applies to every Zombie type – if they’re killed in a way that doesn’t damage their head very much, their Headcrab can pop off and attack the player itself even after the body is dead. The other, which only applies to regular Zombies (and Fast Zombies in the Half Life 2 episodes), allows them to survive explosions or bisection as a crawling torso.

Zombies can also “sleep” by lying down or against a wall, waking up when a player passes by or they take any damage. This is mainly used to scare players or keep them on their toes in areas full of Zombie corpses. In addition, when set on fire, they’ll continue walking after the player rather than stopping and panicking like other NPCs, but will still eventually die if the fire persists long enough to drain their health.

The Zombie As An Entity

As an entity, NPC_Zombies are one of the most basic functional NPC types in the engine, with no unique Flag options or settings. They also don’t respond to tactical options very well – while they’ll still follow Squad rules or other similar settings, they won’t be able to carry out the goals they’re given nearly as well as more intelligent NPCs.

All of their unique attributes and abilities are built into the entity itself, so you don’t have a way to easily turn them on or off without altering the entire NPC_Zombie entity. There’s also nothing particular special in their Inputs and Outputs, with almost all of the options being shared across all other NPCs.

Despite it’s intentionally limited behaviour as an AI, it still responds well to navigation meshes, paths and other movement-based interactions. Like any other NPC, they won’t head towards you in a straight line, so a good navigation mesh will allow them to get through complex areas without getting stuck.

The Zombie As A Level Design Element

Zombies are the most basic living threat a player can come across, which makes them far more limited in terms of how they can be used in mapping. However, it also opens up a lot of interesting ways to experiment with them, especially if you’re able to turn them into a genuine threat.

If there’s one thing that sets Zombies apart from regular enemies, it’s the way they respond to cover and obstructions. Against normal Zombies, cover is practically meaningless to the player, since it restricts their view and makes it much harder to keep track of each individual threat without reducing the danger they’re in. Zombies, obviously, can’t fire back at you, so cover won’t do you any good – there’s nothing to take cover from.

A single zombie is a decent match for a player with no weapons or nothing but a melee attack, but becomes irrelevant once they have a ranged weapon. Ammunition scarcity can be a good way around this, especially if the player has to decide how much ammunition they can afford to waste killing a Zombie that might never actually get in their way. This is the single most useful factor when you’re designing good Zombie encounters: limited resources. Limited space, ammunition, health or time can all turn a simple low-level threat into something much more dangerous, and even a fully-equipped player can be given a fair fight if they’re not given any high ground to work with.

On the other hand, they make great “filler” for maps where you want the player to expend ammunition without necessarily being in constant gunfights. Standard Zombies are psychologically threatening thanks to their excellent sound design and stiff movements, and players might even enjoy finishing them off when it’s not necessary as target practice. Depending on a player’s personality, they might even aggressively attempt to hunt down all zombies in the area, which can make them a good way of eating into the player’s ammunition reserves and forcing them to play more conservatively.

On the other hand, an infinite swarm of regular, slow Zombies (such as in Half Life 2’s Ravenholm section) can do the exact opposite, persuading careful and exploration-happy players that they need to keep moving and should focus on avoiding enemies rather than trying to take them all out.

Summary

Overall, Zombies struggle to be a threat if the player has ranged weapons, is a fair distance away, is in an unreachable area or can easily evade them by moving around a large open area. However, if they’re used correctly, they can become a major threat that’s able to hound players into certain areas or deny certain pathways to them, and can exhaust their ammunition supplies thanks to their hardiness.

They’re not the most dangerous enemy type, nor are they the most intelligent or varied, but they make up for it with the fact that they’re a very simple, single-minded hostile presence that will always try to close in on a player, making it very likely that they’ll get backed into a corner.

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