Abyssal Princes / Lords
Abyssal princes and lords are beings of staggering power, demonic entities that have grown in power through eons of suffering, both given and received. These tanar'ri are each unique beings, having evolved past the 'shared' forms of their fellows.
Tanar'ri Physique: Abyssal princes and lords are tanar'ri - or have perhaps evolved past such origins. They are each unique in appearance and powers, though they nonetheless share the supernatural abilities common to all tanar'ri, including the following:
* 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 Abyssal prince or lord.
* 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 Fire, Electricity, Heat, and Poison (s): conventional tanar'ri possess complete immunity to these damage forms. An Abyssal prince or lord may switch this up when they emerge, however, changing one invulnerability for another to better fit their new powers.
* Portal Sense (w): planar beings all, tanar'ri 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 tanar'ri can spy them with ease, doing so with intensity 8 ability.
* Resistance to Cold Attacks (s): while not immune to the cold, Abyssal princes and lords are nonetheless quite resistant to it. When exposed to cold damage, these beings benefit from intensity 8 (+2) resistance to such attacks and environmental phenomenon.
* Resistance to Magical Attacks (s): while not immune to all magical assaults, Abyssal princes and lords comes pretty darn close. They benefit from intensity 21 (+5) resistance to sorcerous attacks of all types, making them incredibly hard to put down.
* Resistance to Physical Attacks (s): though they're not quite as proof against magic as they are conventional assault, Abyssal princes and lords can take a whole lot of punishment. They benefit from intensity 12 (+3) resistance to physical attack.
* 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. *
Causality Control (i): those Abyssal princes who take command of a layer (or more) of the Abyss gain a limited form of this power. They cannot wield it directly (say, against foes), but they can use it to slowly reshape layers they have conquered as they see fit.
While in their layer, an Abyssal lord can change any aspect of the universe to suit their purposes, from gravity to molecular makeup to even metaphysical properties, if they wish. Which is why so many layers of the Abyss are so horrible to visit - much less live in.
Revival (s): like their lessers, Abyssal princes and lords can sometimes give death the laugh. Unless killed inside the Abyss or eaten, they may attempt to reconstitute themselves on their home plane with intensity 10 ability. The difficulty involved, of course, depends on how they die.
Unique Powers: while the Abyssal princes and lords generally possess the same 'pool' of powers common to all tanar'ri, that's where the required similarities end. Thanks to their unique natures and power levels, these tanar'ri may have any power set they choose.
When they ascend to the status of prince, a tanar'ri can pick its own form and powers, the two of which most often work together seamlessly. These can involve the usual fangs, claws, and supernatural powers, but can go any direction the prince wishes. Which is kind of the point.
Hindrances / Augmentations:
Susceptibility: like all tanar'ri, Abyssal lords and princes 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.
Nothing specific. An Abyssal prince or lord may have a unique item that they have crafted that is either an extension of their own life force and power, or was built separately over eons of time to increase their might. Or they may just carry personal items from times and forms past.
Nothing specific. One of these powerful tanar'ri is likely to hold one or more skills relevant to their history, or perhaps one of the forms they inhabited in the past. Alternately, they may possess other skills related to their final ascension beyond the 'common' tanar'ric shapes.
World Domination: while there are as many differences in callings as there are tanar'ri princes and lords, the most common motivator to these beings is the acquisition of ever more power. The stronger one of these beings is, the less likely someone else's Answer will dominate their own.
The Abyssal princes and lords, they are chaotic and evil like all tanar'ri. However, they are smart enough and powerful enough that they can hold in their overwhelming hatred and madness when it suits their purposes. This allows them to lead other tanar'ri (and not just kill them).
Many are imperial, and many more are murder embodied in physical form - and many more are both such things. However, having earned their place in the cosmic scheme of things, these entities are not bound by any precepts save their own history, personality and philosophy.
Just like the personalities of the Abyssal princes and lords, their appearances can be literally anything. They may appear as a 'super' form of a normal tanar'ri, a 'modified' representative of another race, or even something as strange as the most alien of hordlings.
An Abyssal prince is a tanar'ri who, after uncounted eons, has evolved past the known, common forms of their kind. They've increased their personal power such that they no longer need to lean on such a crutch, and can chose a unique form and power set all their own.
As such, an Abyssal prince will generally have the same 'common' powers that other tanar'ri do, but this is not always the case. For instance, instead of immunity to fire, an Abyssal prince may instead be immune to cold, or perhaps darkness, and wield it like a weapon all the same.
The princes of the Abyss gather on the first layer of the plane, the Plain of Infinite Portals. There, they build armies and fortify whatever stronghold they occupy (or build) on that desiccated ruin of a universe, searching for a lower layer they can call their own.
This involves either conquering a layer for their own use, or making a deal with it to co-rule; each layer of the Abyss, while not necessarily smart, is definitely aware. Those who fail are absorbed into a layer, and added to its power (making it even harder to conquer).
Those rare princes of the Abyss who can manage to conquer one (or more) layers of the plane are known as Abyssal lords. They gain the ability to reshape their layer in any way they can imagine - and the tanar'ri, being creatures of chaos and evil, can imagine a lot.
They actually gain a limited form of the causality control power, letting them rewire the physics (and metaphysics) of their Abyssal layer, working in concert with the layer itself to make it reflect the lord's mind and desires. And now you know why the Abyss is so horrible...!
Thus, the Abyssal lords hold sway over the tanar'ri - or at least, the portion of that race that occupies the layer they command. Most Abyssal lords work hard to further the race's overall goal, that of keeping the baatezu from dictating their beliefs on the tanar'ri as a whole.
Thus, they participate in the Blood War, as much as they can afford, at least when not fending off attacks from other lords or would-be lords, or dealing with meddlesome adventurers. Which makes them the most likely creators of the molydei - who else would create tanar'ric police?!
* Depending on which version of the Planescape lore you wish to use, the Abyssal princes and lords (along with the rest of the tanar'ri) may not have access to this power any longer, having lost it due to yugoloth meddling. Its presence is at the discretion of the individual Narrator.
Of course, one could imagine that these beings have figured out how to make space and time their playthings, being such powerful creatures of chaos, and have these powers despite the plotting of the hated yugoloths. But that may of course depend on the nature of each such individual.