Imagine an entire race of creatures - whose numbers are unimaginabley vast - whose minds are filled with nothing but hatred, death, and vile contempt. Imagine beings so dark as to make the blackest pitch of night seem bright by comparison. Imagine the tanar'ri.
The Abyss is an infinite, empty place of despair. It is the disgusting toil and labor of existence made tangible. Infinite layers of bleak despair are stacked endlessly atop one another in the Abyss, chaos and hopelessness reigning everywhere. That is the home of the tanar'ri. There, where others linger in endless torment, the tanar'ri thrive.
In the broadest terms, tanar'ri are chaotic and evil. The less intelligent often attack without question and fight until slain. True and greater tanar'ri often roam the Astral and Ethereal planes - their attention is attracked to persons in an ethereal state.
In addition to the separate characteristics and abilites of each sort of tanar'ri, they all share these spell-like powers:
They also can gate in large numbers of other tanar'ri at will, as defined for each individual.
All tanar'ri have a form of telepathy that enables them to communicate with any intelligent life form, regardless of any language barriers. Tanar'ri with average or better intelligence are able to converse.
Most of the time, the Abyss-forged magical weapons of the tanar'ri dissolve upon the death of the creature. When one doesn't, it is probably that the weapon in question originated elsewhere.
There are five different varieties of tanar'ri, each with its own particular niche in the Abyss. They are, listed in order of ascending power; least, lesser, greater, true, and guardian.
These classifications of the tanar'ri actually mean very little in their lives. They are merely broad estimates of their destructive power. The tanar'ri are not subtle and have little use for anything besides power, and a strong lesser tanar'ri who bests a weaker greater cousin is awarded greater status in the Abyss. Such petty battles of political positioning are virtually endless on the plane. The only exception to these endless power struggles is the molydeus, or guardian tanar'ri that seem to be curiously outside of the tanar'ri power structure.
As fiends, the tanar'ri are involved in the ongoing Blood War. In fact, they are one of the two major antagonists of that ancient war. For as long as the tanar'ri have existed, they have waged war against their ancient enemies, the baatezu.
There are vast differences in the way that the tanar'ri wage war as opposed to the baatezu. The baatezu are an organized lot, taken to fighting their battles with form and structure and relying on strategy. The tanar'ri, however, are the opposite. They are a horde of chaos and disorder, tending to use their great numbers to wage wars of attrition. In their favor, the tanar'ri have almost endless numbers to draw upon. It is difficult to estimate how many tanar'ri there actually are considering they inhabit an infinte number of infinitely sized planes, but there are easily 100 or more times as many tanar'ri as there are baatezu.
The only reason that the Blood War is waged at all by this seemingly discorded race is that the true tanar'ri seem to have a primal urge to wage the battles and thus force those less powerful than themselves to comply with their wishes.
Tanar'ri are a subset of the far larger race of fiends, or lower planar creatures of supernatural power, according to the definitions made by mankind. It is fairly interesting that sages delineate fiends with a definition that would be meaningless if mankind did not exist, for fiends doubtless do not consider their abilities to be supernatural.
Most tanar'ri feed on either the flesh or the life force of other living beings. It appears that they can derive more nutrition from a victim by instilling more terror in it before the kill. Whereas most predators simply stalk and then kill, tanar'ri have a third step; stalk, terrify, kill.