The monstrous chasme are tanar'ri, demonic beings hailing from the Abyss. They are fell creatures of belief made flesh, supernatural horrors that have a variety of powers - some magical and nature, and some due to their somewhat insectoid physiology.
Tanar'ri Physique: the teeming masses of chasme are greater tanar'ri, powerful demonic creatures that hail from the infinite planes of the Abyss. Like all tanar'ri, the chasme are truly planar entities, and possess these tanar'ric capabilities:
* Darkness: all tanar'ri have the ability to share the inherent darkness of their very souls with the world at large. This rudimentary power functions at but rank value 4, but basically 'blacks out' the sector a chasme currently occupies (it works within Near range).
* Damage Reduction / Cold and Magic: though they are not immune to the effects of cold or magic, chasme are considerably resistant to these attack forms. When exposed to any such damage, a chasme may reduce its intensity by 2 RS - before any other defenses apply.
* Dimensional Transit: in addition to being able to teleport, tanar'ri can transition themselves between planes of existence. They may do so with rank value 10 ability, though on a failed ACT roll dimensionally transiting tanar'ri may be subject to error, per a failed teleport. *
* Infravision: many of the Abyss' countless layers are dark beyond mortal ken, and have never seen the light (physically or otherwise). As such, all tanar'ri have evolved the ability to see by heat as well as by visible light, and can do so with rank value 10 ability.
* Invulnerability to Fire, Electricity, Heat, and Poison: the tanar'ri are hardy creatures - they have to be to survive at all in the endless, horrific wastes of the Abyss. Their forms have evolved over time to give them complete immunity to fire, electricity, heat, and poison.
* Portal Sense: planar beings all, chasme have the ability to perceive the various rifts that dot the planescape of the multiverse. Most of these doorways are invisible to the naked eye, but all chasme can spy them with ease, doing so with rank value 20 ability.
* Telepathy: almost all tanar'ri are able to communicate directly from mind to mind - a vital skill when so few speak the same tongue. They can usually broadcast and receive thoughts with rank value 6 skill, enough to get their point across when necessary.
* Teleportation: chaotic creatures all, the tanar'ri can bend space itself to their will. All tanar'ri possess the power to teleport with rank value 30 ability, and can avoid teleportation's usual side effects while transporting themselves within the same plane of existence. *
Additional Limbs / Wings: chasme, resembling a hideous fly or mosquito of some sort, possess insectoid wings. These appendages bear them aloft with rank value 6 speed, allowing them to fly at up to 81.81 miles per hour given proper air conditions.
Attenuation: this dangerous ability allows the chasme to enfeeble their opponents. If its target fails a Brawn ACT against this rank value 10 power, the chasme can reduce that foe's Brawn score by up to ten points (with a minimum value of 2) for 1d10 turns.
Body Armor: chasme possess a strong chitinous hide, one which makes them somewhat resistant to the rigors of Abyssal life. They possess this physical power at rank value 4, which offers them like protection against physical attacks (but nothing against other forms of damage).
Claws: to soften up foes, chasme often swipe them with their claws. Their four rear limbs each have a claw, with which chasme may inflict their Brawn value in Slashing damage. These claw wounds are particularly severe, and inflict SD MB damage from bleeding.
Emotion Control / Fear: chasme possess the supernatural power of fear, augmented by their monstrous appearance. Anyone within Near distance of this rank value 10 power must pass a Willpower ACT against it after gazing upon a chasme or flee for 1d10 turns.
Nonapparent Vision: the chasme have a limited form of this power, which allows them to perceive items that have been rendered invisible to conventional light spectra. This power works at rank value 10, complementing their infravision quite nicely.
Philosophical Sense: chasme have an innate ability to 'read' the philosophical bent of other sentient beings. This rank value 10 power allows them to sense if a target is either good, evil, orderly, chaotic, or balanced in mindset, and then react accordingly.
Proboscis: chasme do not have fangs; instead, they have a long proboscis with which to puncture their foes and drain their blood. Chasme inflict their Brawn value in Slashing damage with this organ, along with like MB damage in blood loss each turn until the chasme stops feeding.
Sleep: the soft buzzing sound of the chasme's wings lulls its foes into a hypnotic sleep. Anyone hearing the chasme's buzz (within Near range) must pass a Willpower ACT against this rank value 10 power or fall asleep for 1d10 turns (only works once per encounter).
Summoning: the chasme have several summoning powers. Three times a day, they may attempt to summon forth fellow tanar'ri. A red ACT brings forth 2d10 manes, a blue ACT summons forth two to five cambion, and a yellow ACT roll will make manifest another chasme.
Alternately, a chasme may summon forth a hideous cloud of biting, stinging insects. This cloud will occupy the entire sector it is summoned within, and inflicts rank value 10 damage in total to unprotected targets for 1d10 turns (any protection from physical attack will negate this damage).
Telekinesis: chasme may impose their desires upon objects in its vicinity, an advantage since their reach is generally rather poor. They may only do so with rank value 10 ability, but this allows the creatures to move 400 lbs of matter just by thinking about it!
Limitations / Enhancements:
Susceptibility: like all tanar'ri, chasme are weak against the use of cold-wrought iron weapons, as well as holy water and weapons. Their supernatural defenses are ineffective against such assaults, which inflict at least a quarter of their maximum Health in damage.
Unattractive 3: the chasme are hideous inhuman monstrosities! Humanoids react very negatively to chasme, even before you consider their fear powers, and chasme are at a -6 RS to all NPC reactions when interacting with humanoid beings of any kind (even other tanar'ri).
The chasme are considered cowardly for avoiding the front lines, but they nonetheless have the run of the Abyss in their hunt for deserters. They cheerfully return the hatred other tanar'ri bear towards them, and murder their brethren savagely upon finding them to be guilty.
Chasme are hideous and inhuman, resembling a cross between a human and a fly or mosquito. They are primarily insectoid, with a sickly yellow color to their chitin, a vaguely humanoid face, and two humanoid arms as forelegs (the other four limbs are insectoid, and bear claws).
Chasme have tiny, vestigial mouths with which they can attempt raspy speech, but mostly communicate via telepathy. The most notorious feature on a chasme is its proboscis, a needle-like nose / mouth extension with which it may puncture its victims and feed upon their bodily fluids!
The teeming masses of chasme relentlessly scour the Abyss in search of Blood War deserters, who they kill slowly and mercilessly. They do this not out of loyalty but to avoid being sent to the front lines themselves; for all their power, chasme are cowardly and mercurial monsters.
Other tanar'ri loathe the chasme for their appointed task, readily slaying them when they think they can get away with it - or even when they don't. The only thing keeping chasme from annihilation in the Abyss is the fact that they breed prodigiously.
While some chasme are a result of weaker tanar'ri ascending to this 'station' on their rise to power, most chasme come about from eggs lain by other chasme. An adult lays ten eggs per month, which hatch after three months of development, and grow to adulthood in but seven years.
Assuming they last that long. For while the chasme are incredibly powerful, they do live in the Abyss - and if other tanar'ri don't kill them, there's any number of other threats with which a young chasme may meet its end. Chasme die with some frequency, but they have great numbers.
* Depending on which version of the Planescape lore you wish to use, the chasme (along with the rest of the tanar'ri) may not have access to these powers any longer, having lost them due to yugoloth meddling. Their presence is at the discretion of the individual Gamemaster.