Description and Biology
We are told that the Gelth were originally regular organic beings. They somehow lost their physical bodies during the Time War, becoming gaseous instead. How this would work is obviously unclear, but, in fairness, we have little idea from the TV series of how exactly the Time War was fought or what the weapons brought to bear in it were capable of.
Either way, the Gelth as they are now are gaseous wraiths, capable of creating partially humanoid shapes, but generally not bothering to do so. The evidence suggests that are composed of organic gases; they can survive without difficulty in natural gas, which is mostly methane, so it’s unlikely they contain much of that or they’d diffuse away, but ethane, propane, butane, and more complex hydrocarbons may all be involved. Biologically, it’s hard to see how this could possibly work, at least in Earth-like conditions, so a more likely explanation is that the body is somehow held together by force of psychic will.
Which is basically saying “it’s magic”, rather like the air elementals of fantasy games, but it’s what we’ve got.
The Gelth can control and move their gaseous substance around which, in the context of the story, means that they can inhabit the putrefying gases inside a human corpse and make it move around. Their preference for humans here, since any other animal would surely also work, is likely a simple matter of availability, given where they happened to appear. Assuming that they were originally humanoid, as the briefly-seen blue form implies, there’s probably also an improved degree of familiarity with the form that makes it easier (and arguably more useful) for them to animate a human cadaver than, say, a dead rat.
We know essentially nothing of Gelth society and culture, beyond the fact that it was destroyed in the Time War. We don’t know whether they breathe or how they obtain energy although whatever they do it’s likely not obvious – obtaining energy from the air molecules around them, say. Lacking any actual sense organs, we’re again left with psychic powers to explain how they navigate the world around them; perhaps the ‘real’ Gelth are purely psychic constructs that find it easiest to interact with the environment by controlling gas molecules. Although, if so, the molecules must act as a necessary tether, since destroying them (by combustion) kills the Gelth and prevents it from reforming.
How, and even if, Gelth reproduce is equally mysterious. Potentially, it could be an ability that they lost when they lost their bodies, although with “billions” of them supposedly still in existence at the time of the TV story, that must have been a recent event for them – for whatever that means in the Time War.
Game Attributes
While inhabiting human bodies, the Gelth are essentially zombies, a concept familiar in many fantasy games and echoed a few other times in Doctor Who itself. We can see that they move slowly and clumsily, doubtless due to the difficulty of the Gelth controlling the host body, and there’s no evidence to suggest that they are any physically stronger than they were in life. They likely are physically resilient, however, being unable to bleed to death and having no vital organs since they are animated purely by the putrefying gases inside them. Similarly, they won’t feel pain, since even if their nerve endings are still functional (which seems unlikely) the Gelth isn’t actually inhabiting
their nervous system and wouldn’t receive any signals from it. Destroying the zombies is instead a matter of somehow getting the gas to leave the body or destroying enough of it that it’s no longer functional.
Depending on the game system, this could be quite difficult or just a case of knocking a few hit points off.
The Zombies seem unintelligent, partly because the Gelth don’t seem to have enough control of their vocal cords to be able to speak. Logically, this is something they could get the hang of with time, since gas moving about is rather how speech works, but gaining the necessary fine control evidently isn’t a simple process. Nonetheless, while it may appear stupid, the zombie is directly controlled by a Gelth, and these are presumably no less intelligent than the average human – and their leaders likely more so. Some remnant of the original soul apparently remains, but the Gelth seem to be able to force it down with a little effort.
There’s also the question of the Gelth in their natural form. Here, they appear agile but have a physical strength no more powerful than a breeze, likely at the very minimum that a given system will allow. How difficult they are to destroy depends on how strongly the animating force can hold the molecules together and, from everything we see, that’s pretty tough. Physical attacks aren’t going to do anything to them, so we’re left with psychic assaults and effects that might change their chemical composition. The obvious example here is combustion, since we know they are flammable, and this seems a particularly high vulnerability. (When it comes to D&D, it’s worth noting that lightning attacks would probably also ignite them).
They probably can’t be poisoned, even with other gases – unless, perhaps the ’poison’ reacts with their hydrocarbon molecules to make something they can’t control. But this isn’t necessarily an issue in plausible conditions. For example, while hydrocarbons react with chlorine, the result is usually still a gas, which the Gelth may not have any problem animating… although they’re likely to smell worse as a result.
Lack of any knowledge of their culture makes it harder to say what skills the typical Gelth might have. Being so silent, they’re going to be stealthy and they can evidently sense their environment and communicate with anyone who is psychic, but otherwise we don’t see them doing anything that’s indicative of what they get up to. Possibly not much, lacking bodies as they do.
Other than psychic communication, their only special power is the ability to possess corpses.
5E - Gelth Zombie
Medium undead, neutral evil
Armour Class: 8 (natural)
Hit Points: 34 (4d8+16)
Speed: 20 ft.
STR 10 (+0) | DEX 6 (-2) | CON 18 (+4) |
INT 10 (+0) | WIS 10 (+0) | CHA 6 (-2) |
Damage Resistances: piercing, bludgeoning
Damage Immunities: poison
Condition Immunities: poisoned
Senses: Passive Perception 10
Actions:
Unarmed. Melee
Weapon Attack: +2 to hit, reach 10 ft., one target. Hit: 1 bludgeoning
damage.
Challenge: 1/8 (25 XP)
5E - Gelth
Medium elemental, neutral evil
Armour Class: 18 (partially insubstantial)
Hit Points: 26 (4d8+8)
Speed: fly 60 ft.
STR 1 (-4) | DEX 16 (+3) | CON 14 (+2) |
INT 10 (+0) | WIS 10 (+0) | CHA 10 (+0) |
Skills: Deception +2, Stealth +5
Damage Resistances: cold, poison, thunder
Damage Immunities: acid, bludgeoning, piercing,
slashing
Damage Vulnerabilities: fire, lightning
Condition Immunities: grappled, paralysed, prone,
restrained
Senses: Passive Perception 10
Actions:
Possession. As
an action, a Gelth can possess a recently deceased corpse, creating a Gelth
Zombie, which becomes active at the beginning of the next round. If the zombie
falls to 0 hit points, the Gelth is released unharmed, but then becomes
vulnerable to external attacks.
Challenge: 1/4 (50 XP)
BRP - Gelth Zombie
STR 3D6 (10-11) | CON 2D6+9 (16) | SIZ 2D6+6 (13) |
INT 2D6+6 (13) | POW 3D6 (10-11) | |
DEX 2D6 (7) | CHA 2D6 (7) | |
Hit Points: 15 | Move: 8 | |
Base SR: 6 | Damage Bonus: +1D4 |
Armour: None
Skills: Grapple 50%, Stealth 30%
Combat Skills: Brawling 50%, damage: 1D3+1D4
BRP - Gelth
STR 0 | CON 2D6+9 (16) | SIZ 1D6+6 (9-10) |
INT 2D6+6 (13) | POW 3D6 (10-11) | |
DEX 2D6+9 (16) | CHA 3D6 (10-11) | |
Hit Points: 13 | Move: 15 | |
Base SR: 7 |
Armour: None
Skills: Dodge 80%, Fly 70%, Persuade 50%
Possession: A Gelth can possess a recently deceased corpse, taking one round to enter and animate the body. This creates a Gelth Zombie, which becomes active at the beginning of the next round. If the zombie is destroyed, the Gelth is released unharmed, but then becomes vulnerable to external attacks.
GURPS - Gelth Zombie
ST 10 | DX 6 | IQ 10 | HT 14 |
Thrust: 1d-2 | Swing: 1d | ||
Speed: 5 | Move: 3 |
Advantages: Doesn’t Eat or Drink, Doesn’t Sleep, High
Pain Threshold, Injury Tolerance (Unliving)
Disadvantages: Appearance (ugly), Bad Smell, Cannot
Speak, Compulsive Behaviour, Unhealing
Skills: Brawling-10, Stealth-8, Wrestling-10
GURPS - Gelth
ST 0 | DX 16 | IQ 10 | HT 14 | |
Thrust: 0 | Swing: 0 | |||
Speed: 7.5 | Move: 7 | |||
Size: -2 | HP: 10 | |||
Advantages: Body of Air, Doesn't Eat of Drink, Doesn't Sleep, Enhanced Move-1 (Air), Possession (parasitic, dead bodies only), Telecommunication
Skills: Acting-12, Persuade-12, Stealth-18
Savage Worlds - Gelth Zombie
Agility: d6 | Smarts: d8 | Spirit: d8 |
Strength: d8 | Vigour: d10 |
Skills: Fighting d4, Stealth d6
Hindrances: Mute, Slow (major), Ugly
Special Abilities: Undead
Pace: 4 Parry:
4 Toughness:
7 Size: 0
Savage Worlds - Gelth
Agility: d12 | Smarts: d8 | Spirit: d8 |
Strength: 1 | Vigour: d10 |
Skills: Persuasion d6, Stealth d8
Edges: Dodge
Powers: Possession (can possess a recently deceased
corpse, taking one round to enter and animate the body. This creates a Gelth
Zombie, which becomes active at the beginning of the next round. If the zombie
is destroyed, the Gelth is released unharmed, but then becomes vulnerable to
external attacks.)
Special Abilities: Elemental, Flight, Gaseous Form
Pace: 8 Parry:
2 Toughness: 6 Size: -2
STA - Gelth Zombie
Control: 6 | Fitness: 12 | Presence: 7 |
Daring: 10 | Insight: 8 | Reason: 8 |
Command: 0 | Security: 1 | Science: 0 |
Conn: 0 | Engineering: 0 | Medicine: 0 |
Stress: 13 | Resistance: 0 |
STA - Gelth
Control: 11 | Fitness: 9 | Presence: 10 |
Daring: 10 | Insight: 8 | Reason: 8 |
Command: 1 | Security: 1 | Science: 1 |
Conn: 0 | Engineering: 0 | Medicine: 0 |
Stress: 10 | Resistance: 0 |
Gaseous: Cannot be injured by physical attacks; can
enter a recently deceased body and possess it, creating a Gelth Zombie
