Outsiders are spoilers by definition. Players, don't read.
Arcana DC 15
Outsiders are creatures that come from outside the world. Angels (of Pelora) are not outsiders. Devils (of Ashmadeva) are not outsiders. Gods are not outsiders. Outsiders are evil creatures like Demons or Elementals who desire to destroy the whole world. Angels fight against them.
Arcana DC 20
Outsiders are creatures that come from outside the world. Generally, there are two types: Astral Creatures and Elementals. Demons are a kind of elemental, arising out of the Abyss where Tharizdun is imprisoned. They tend to be more intelligent than most elementals of comparable power, so a lesser demon would be smarter than a lesser elemental, and an archdemon would be smarter than an archomental (also known as a Primordial). Elementals are pretty easy to fight, since you can just beat them up until they crumble into dust/ash/water/smoke.
[Digression]
Demonic Cults are not uncommon, though certainly less common than Diabolic cults. Said cultists are often revolutionaries, cowed servants of some pseudo-immanent archdemon, or the foolish rabble that buy into the latest Tharizdun-based Heresy. They are very big into sacrificing the innocent and immanentizing the eschaton, as you might expect. The more advanced cults don't tend to go for the whole "plunge the curvy dagger into the beating heart" thing, prefering a ritual which vaporizes the sacrifice, body and spirit.
[/Digression]
Or at least they are easy compared to creatures of the Astral Sea (also known as the Far Realm, the Deep Heavens, etc.). Just looking at one can make you go mad, and often they are intangible or otherwise resistant to beating up. For some reason, they tend to be considerably tentacular. They may be combated with the Cerulean Sign, an arcane glyph that can stun or even banish them.
Angels, that is, proper Angels, also called Outer Angels, Primordial Angels (not to be confused with Primordials), and Birds (if one is speaking cryptically) are mainly concerned with fighting Astral creatures, not Elementals, which is normally the job of gods and their servants. However, some gods have a working relationship with the angels, and are able to request things of them. Namely, Pelora, Avandros, Ioun, and the Raven Prince are said to be on good terms with them. No one knows who created the angels. Some theorize they arose spontaneously, a creative and benevolent primordial counterpart to the Elementals. Others believe they appeared when the first Demons did, for similar reasons. They are faceless, glow brightly, have wings, and cannot lie.
There are other outsiders. Many have been spirited to a strange city, known as Sigil, which lies outside our world, between the Elemental Chaos and the Astral Sea. Wizards say it is the only other world in which people can survive for any period of time. Its only native inhabitant is the Lady of Pain, a silent and imperious presence who occasionally deigns to punish interlopers. They say she hates the gods, if it may be said that she feels anything, but may sometimes tolerate humanity.
Arcana DC 25
Outsiders are creatures arising from outside the world. Generally, there are three types: Astral Creatures, Elementals, and Angels.
Astral Creatures, also known as Stars or Elder gods are beings from the Astral Sea. They are creatures of thought and creativity far beyond what humans, or even gods, are capable of. They lie in contrast with the Elementals, beings of matter and destruction, though of course the Elder Gods have bodies (of a sort) and the Elementals have minds (of a sort). He Who Was, if Ioun is telling the truth, was an elder god, inventing by some eldritch process the many and varied concepts of our world. Ioun is one as well, and some hypothesize that Tharizdun might be one. Experience tells us that there are elder things much less powerful than the gods, perhaps servants or offspring of the greater elder gods.
The danger of the elder things is not mainly in their power, however. Just as He Who Was invented concepts like good and evil, colors and sounds, the elder gods invent concepts just as basic, just as obvious, yet totally alien. Further, in the words of Ioun, "The people of the starry city like to use fashion to express themselves". The appearance of even the lowest elder thing perfectly communicates the alien ideas that can overwhelm a weak human psyche like kudzu. The common idea that they are mostly tentacles describes only the one's least likely to fry your brain.
The Stars are generally too far away to be seen in any detail, however. Only with a powerful telescope is one in danger from the very nearest stars: Hadar, Delban, Gibbeth, and the rest. Light, almost a synonym of existence, appears to be one of the fundemental concepts of even the astral sea. Thus, all living stars shine with the light of their hyperreality. The stars, it is said, bring fevered dreams on moonless nights, which are blessedly forgotten by morning.
Birds, also known as Angels, are a mysteriously benevolent type of outsider. They may fly anywhere, and may be found everywhere: the freezing darkness of the Astral Sea, the depths of the Abyss, the city of Sigil... Every bird is an angel. Many birds that you commonly see are very little and weak angels, but they are still angels*. Some of them are in fact greater angels in disguise. The physical forms of birds were made by the gods, to reduce the memetic effect of angels on the world, apparently at the angels' own request.
(*The exception to this is chickens, which are in fact a type of genetically engineered bat. Ashmadeva made them out of spiteful humor.)
Various types of lesser birds have especial agreements with certain gods. Doves help Pelora spread peace. Songbirds help Avandros to inspire artists. Owls observe and collect on behalf of Ioun. Ravens, obviously, work on behalf of the Raven Prince (this agreement actually precedes the Raven Prince's godhood, which is why he is named after them.)
It was the Angels that developed the Cerulean Sign, in partnership with Ioun and the Lady of Pain. Its effect on astral creatures is something like a complex riddle, something like a legal document of protection, and something like pepper spray.
The Lady of Pain, is, of course, an elder god, just like He Who Was. Allegedly, she spoke with the Angels and asked them to defend the world. Sigil is her world, made after the death of He Who Was. Sigil is not the name she gave it (for she never speaks). It is the name people gave it, after the numerous heiroglyphics that cover the entire small world. It is also called the Stone Library, the City of Doors, the Maze, the Grey Market, and the Scarred World. The eponymous sigils are, apparently, fractal in nature: each "one" is made up of numerous smaller signs, and each cluster makes up a single discernable larger sign.
Commonly, wizards visit Sigil to study its glyphs and properties. However, to enter purposefully is much rarer than to enter by chance. Most of the people in Sigil entered by walking through an otherwise normal door, and will eventually exit the same way. If this is a purposeful act of the Lady, no one knows the reason.
Viewing the Lady of Pain does not have the same effect as viewing most elder gods. It has an effect, certainly (namely that, while viewing the Lady, it is impossible to recall seeing anything else. Some describe feeling like they were tortured for an eternity by the sight, but they are often none the worse for it). However, the ideas of the Lady were present at the beginning of the world, and are thus endemic to the human spirit. If He Who Was still lived, venture the scholars, then viewing Him would similarly have no maddening effect.
Next section is a major spoiler, and if my prospective players read it, the Lady will flay you.
Arcana DC 30
This is the heresy which all the gods detest
Tharizdun is called the chained god, and the father and mother of demons. To follow Ioun's example: When the Lady killed He Who Was, after his two sons emerged, his body became Tharizdun (who, perhaps, was god of Nothing). The children of He Who Was feared Tharizdun, or were ashamed, and he was banished, or imprisoned, or entombed in the Elemental Chaos. It was the dregs of this colossal impiety, this cosmic dishonor, that corrupted the Abyss and spawned the demons, not the evil of Tharizdun.
But the Angels fly even to the depths of the Abyss, and this is what the six-winged bird, the Angel of the chained god, has said: "He is returning."
"He has tested good and evil. He has tested war and peace. He has tested life and death. And He has found His answer.
He has spoken with Angels, and they have heard Him. He has spoken with Fire and Water, Air and Earth, and they have heard Him. He has spoken with His children, and they have not heard Him.
He is returning, and He shall end the War, and judge all things, and then shall go to the Eldest God and speak to Him a new thing, and there shall be no more Death. Let him who fears descend into His grave to speak and to strive. His fool is His herald, and shall reveal good and evil, and shall gather seven that He has lost. The great angel of the outer darkness shall watch the world until He comes. Let the Four, His sons and daughters, descend to meet Him before THE END."
