Z-Sec

The Z-Sec (Zombie-Security) is a closer recreation of the classic zombie, as they carry firearms; Doom 3's civilian zombies do not. Unlike the classic variations, all Z-secs wear black uniforms and armor, possibly in a reference to the classic shotgun guy.

Z-secs were originally human security guards, perhaps marines assigned to sentry duty, that were possessed and mutated by evil demonic spirits. Other marines that avoided this fate were soon overrun by demonic forces; one marine has just been strangled to death by a zombie by the time the player reaches him. In trailers and early in the game, an in-game monitor shows one being possessed by a ghostly skull, which causes his skin to turn white and his eyeballs to explode. Several of the Z-secs have their eye sockets hidden due to their helmet visor, although it is assumed that they have lost their eyes. The visor itself is completely red, probably due to the blood. Despite this, they are are still able to "see" the player, possibly due to the hellish spirits controlling them.

There are four types of Z-secs, each with its own distinct characteristics and firearms. These are the Pistol Guard, the Shotgun Guy, The Machinegunner and the Riot Shield Trooper.

Combat Characteristics
Unlike regular zombies, Z-secs do not suffer from slowed movement, and possess the same speed and agility as a normal human soldier. They are also notable for their artificial intelligence (AI), which makes them fight like soldiers. They duck behind objects, lean around or over cover to take shots at the player, and occasionally rush the player's position. When shot, they can sometimes sidestep a single step to attempt to dodge further attacks (this can effectively allow them to dodge a follow-up shotgun blast at close range). However, they do not shoot and move at the same time, leaving them vulnerable to the player's fire when advancing forward from one position to another.

Like most of Doom 3's monsters, headshots do twice as much damage as regular body shots. Compared to their Classic Doom counterparts, Z-secs are more focused and intelligent, advancing towards and focusing on the player instead of wandering semi-randomly. They also have better aim and are a lot more aggressive, firing more frequently. When alerted, they respond in a radio-distorted voice, perhaps a homage to Half-Life grunts (government soldiers).

Bald guy with no helmet and glowing white eyes, armed with a pistol. They have the unique ability to dodge by rolling sideways, and always retaliate with a few pistol shots after a successful dodge. They occasionally fire multiple shots in rapid succession and are generally decent shots, but of all three Z-sec types, these are the ones that are the most likely to miss. They have about half as much health as the other Z-sec types, and take around four pistol bullets to the body to kill. Drops a pistol when killed.
 * Pistol Guard:

Similar to the Classic Doom shotgun guy, but he wears a helmet and a broken visor, and his eyes are missing. He is about twice as tough as the Pistol Z-Sec, but still reasonably easy to kill. Unlike the player's shotgun, the Z-sec shotgunner's weapon has narrower spread, but it is considerably less powerful than the player's version. Can sometimes rapid-fire at a faster rate than the player. Drops a shotgun when killed.
 * Shotgun Guy:

Wears a closed helmet with an intact but bloodspattered visor (obscuring his eyes) and a breather, ultimately covering his entire face. Unlike shotgun Z-secs, instead of rushing the player's position these zombies will often dig in behind cover and aggressively spraying the player's position with bullets. If the player hides behind cover, these Z-secs will suppress them with continuous machine gun fire. Arguably the toughest of the three types. Drops a machinegun when killed.
 * Machinegunner:

The rarest Z-sec type, with only a few being encountered throughout the game. In addition to a pistol, this type carries a large Riot shield and wears a helmet, protecting it from attacks. The shield effectively protects it from anything that hits the shield, and being fully armored himself, he is still fairly tough (similar to the Machinegunner) when hit where his shield does not protect him. AI is the standard Z-Sec combat tactics.
 * Riot Shield Trooper:

Tactical Analysis
Although Doom 3 Z-Secs serve a cannon fodder role when compared to the game's tougher demonic enemies, they are both comparatively tougher than the classic zombie troops and equipped with better AI, and should not be underestimated.

Using cover is extremely important when fighting these zombies, as their hitscan weapons cannot be dodged by circle strafing. The Machinegun is generally a good basic weapon against these pests and a good tactic is usually to mow them down with concentrated pistol or machinegun fire, switching to the shotgun to finish them off if they rush the player at close range. It is usually a bad idea to use the shotgun right away as the shotgun has short range and the Z-Secs are tough enough to potentially survive anything less than a direct point-blank blast, leaving players vulnerable after the initial shot. A headshot with the shotgun will kill a Z-Sec instantly, however.

More powerful weapons like the chaingun and the plasma gun are very effective against Z-Secs but should probably be reserved for tougher monsters or larger crowds of Z-Secs.

The Pistol works well against this type, but the machine gun gets the job done quicker. It is often a good idea to keep a good distance as he is more likely to miss his mark when the player is far away. Due to their low health, the shotgun is also very effective against them even when fired from about a dozen feet away.
 * Pistol Guard:

This type should be brought down as quickly as possible, as he is capable of dealing a great deal of damage if left unchecked. Using the machinegun to soften him up at range and then finishing him off when he gets in close is an effective strategy, but caution should be taken only to come out of hiding after he has fired a shot. More powerful weapons should be considered.
 * Shotgun Guy:

Alternatively, hide behind a doorway when available, shotgun at the ready. Wait for him to charge through the door and blast him in the face. This works 99.9% of the time, but be prepared to fire again if for some reason the first shot did not kill him.

Although he fires more than the other types, his bullets are less powerful. Patience is a good tactic, as he tends to pin down the player with prolonged sprays. It is effective to wait for him to come out of hiding, shoot and quickly duck down behind cover before he opens up. It is a bad idea to fire prolonged bursts at him, as he is tough enough to recover swiftly from hits, often enabling him to retaliate swiftly. Machinegunners do not fire and move at the same time, and so they are vulnerable to return fire when advancing forward towards the player's position.
 * Machinegunner:

Alternatively, you can use the same doorway tactic as with the Shotgun Guy. This tactic works better against Machinegunners than Shotgun Guys because even if he survives the first shot, he will take the time to kneel before shooting you, allowing you to get in a finishing shot.

This type requires accurate aim and cool nerves as his shield is impenetrable. Keeping one's distance is strongly advised if weaker weapons like the machinegun are used. The best way to dispatch of this nasty trooper is run into him with the Chainsaw (it will damage him even when the shield is hit). Another possibility is to hit him with a grenade.
 * Riot Shield Trooper:

Combat Details

 * Health: 55 (pistol), 90 (machine gun and shotgun), 110 (shield)
 * Attack Damage: 4 per round (pistol/shield), up to 28 (shotgun), 3 per round (machine gun)
 * Locations: Mars City Underground - Alpha Labs Sector 4, Communications Transfer, Comunications, Monorail, Delta Labs Sector 3, Erebus Sector 2 - Erebus Sector 4, Phobos Labs Sector 1, Phobos Labs Sector 2, Phobos Labs Sector 1 (revisted)
 * Cheat Spawns:

1. "spawn monster_zsec_pistol" (spawns a z-sec with pistol)

2. "spawn monster_zsec_shotgun" (spawns a z-sec with shotgun)

3. "spawn monster_zsec_machinegun" (spawns a machine gun z-sec)

4. "spawn monster_zsec_shield" (spawns a machine gun z-sec with a pistol and shield)