The wastriliths are powerful water demons hailing from the Abyss. These tanar'ri possess powerful supernatural capabilities, along with a variety of specialized adaptations that allow them to function, and even thrive, beneath the waves.
Tanar'ri Physique: the solitary wastriliths are greater tanar'ri, powerful demonic creatures that hail from the infinite planes of the Abyss. Like all greater tanar'ri, wastriliths are truly planar entities, and possess these potent tanar'ric capabilities:
* Darkness (a): all tanar'ri have the ability to share the inherent darkness of their very souls with the world at large. This rudimentary power functions only at intensity 4, but basically 'blacks out' everything within near missile distance of the wastrilith.
* Dimensional Transit (i): in addition to being able to teleport, tanar'ri can transition between planes of existence. They may do so with intensity 5 ability, though on a failed action dimensionally transiting tanar'ri may be subject to error, per a failed teleport. *
* Infravision (w): 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 intensity 5 skill.
* Invulnerability to Cold, Fire, Heat, Poison, and Water Attacks (s): unlike most tanar'ri, wastriliths are immune to cold, poison and water attacks (water control, etc.). They also bear invulnerability to fire and heat attacks, though only if they are completely submerged.
* Portal Sense (w): planar beings all, wastriliths 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 wastriliths can spy them with ease, doing so with intensity 8 ability.
* Resistance to Magic Attack (s): wastriliths are rather tough creatures. They possess intensity 8 (+2) resistance against attacks that inflict direct magical damage, on top of their normal invulnerabilities to the various attack forms.
* Telepathy (w): 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 intensity 5 skill, enough to get their point across when necessary.
* Teleportation (i): chaotic creatures all, the tanar'ri can bend space itself to their will. All tanar'ri possess the power to teleport with intensity 10 ability, and can avoid teleportation's usual side effects while transporting themselves within the same plane of existence. *
Animal Control (w): though it's nothing supernatural, wastriliths tend to build a 'pack' of followers, mostly due to their being so intimidating. Such a pack will often consist of water elementals, sharks, sahuagin, or ixitachitl - who they send into a fight before entering themselves.
Claws (s): pretty much all tanar'ri have claws, and wastriliths are no exception to this rule. Their long, spindly arms each have digits that are equipped with claws, which allow them to rend their foes with +2 damage in melee (when not biting them, anyway).
Dual Respiration (s): wastriliths are primarily water-borne creatures, and are fully adapted to living beneath the waves. However, they can come up to the surface and attack foes there directly, and are capable of breathing normal air as well as water when necessary.
Fangs (s): the oversized maw of the wastrilith is filled to the brim with long, needle-like teeth. They can use these large implements to inflict their Strength +2 in gnawing damage each turn, assuming they don't simply claw a foe - or hit them with a heat blast.
Growth (a): fell creatures of the deep, wastriliths are rather large. They measure in at around ten to twelve feet long, which means they have this power, after a fashion, at intensity 2. Their Strength is +2 against ordinary sized foes, while their Agility is rated at -2 versus the same.
Heat Generation (i): three times a day, a wastrilith may emit a boiling gout of heat from their fanged maws, with which to crisp most anything in their path. This blast of overheated water can inflict intensity 12 energy damage per deadly attack.
Invulnerability to Memory Alteration (s): an ability few tanar'ri possess, wastriliths are immune to powers and substances that would alter their memories. These include the forgetfulness spell, the mind wipe psionic, and even the waters of the Rivers Lethe and Styx.
Resistance to Pressure Variance (s): while not immune to crushing damage, a wastrilith does possess staggering resistance to it, allowing them to live in seas that would easily mash lesser beings. They can withstand pressure extremes with intensity 20 (+5) ability.
Turnabout (i): wastriliths have this power at intensity 15, where electricity is concerned. It lets them redirect incoming electrical attacks as a contingent action randomly (easy difficulty), at the source (average difficulty) or somewhere else (challenging difficulty).
Hindrances / Augmentations:
Monstrous: it should go without saying, but wastriliths are monstrous entities - just look at them! A wastrilith is at Willpower 0 to prevent Edge 0 humans from fleeing at the mere sight of them - not that they would ever really bother, as any human by a wastrilith is due to be dinner.
Susceptibility: like all tanar'ri, wastriliths 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.
Unlike most tanar'ri, however, wastriliths are also susceptible to fire and heat damage. While immune to such under the waves, a wastrilith caught out of water has no inherent protection against such attack forms, and suffer a like penalty against them.
Peace of Mind: after untold eons of suffering and inflicting pain at the best of others, a tanar'ri who evolves into a wastrilith can finally achieve a moment's peace from the predations of others. Sinking into the depths, wastriliths only want to be left alone with their inner bile.
The wastriliths are brilliant, possibly to the point of instability. They are very powerful, and use this might to ensure their own isolation, so that no one else may try to dictate the Answer to them. Anything foolish enough to enter their territory is cheerfully consumed for fun.
Wastriliths are horrific. They resemble oversized serpents, perhaps ten or so feet in length. They bear two long, spindly arms instead of fins, an oversized head with a maw full of needle-like pincers, and bulging eyes with an almost fish-like aspect to them.
Wastriliths quickly grow to dominate any ecology they impose themselves upon. They tend to choose an isolated undersea lair and lord over all within a twenty five mile radius of it. Anything stumbling into the territory of a wastrilith is eaten, more out of fun than any real need.
They tend to mainly occupy layers of the Abyss which are water-borne, but sometimes find themselves in other planes, where they rapidly prove how dangerous they can be. A wastrilith in the 'real' world will terrorize shipping lanes until it establishes its own, transient 'kingdom'.
Wastriliths do not readily participate in the Blood War, though they will at the behest of Demogorgon, ruler of the 88th layer of the Abyss: the Gaping Maw. Many wastriliths occupy portions of this layer, for it is cold and dark and mostly aquatic, with few minor exceptions.
One step away from true tanar'ri, wastriliths occasionally come about as a result of breeding, but more likely than not occur when weaker tanar'ri finally rally enough power to ascend to their shape. Their hard work over the eons is rewarded with isolation and peace.
At least, peace from the whims of others. A wastrilith usually dictates its own terms to the world, and woe to anything that disagrees with them.
* Depending on which version of the Planescape lore you wish to use, the wastriliths (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 Narrator.