Planar Lore in the Insurrection World

Planar travel is very possible in the Insurrection world, and while still relatively obscure, those outside of underground cults can now practice it. In reality, these planes are not simply other planets. They are entire dimensions with different laws of physics. When something from another plane crosses over, it carries some of those laws with it, therefore leading to a slow, entropic death of the universe.

The Material Plane

This is "the real world". Newtonian physics apply here. Mass coalesces into celestial bodies such as planets. This is where Earth is, and where the main game takes place. The other planes generally interact with each other via the Material Plane, though this is not always true. It is much easier to get into the Material Plane than it is to get out.

The Astral Plane

The Astral Plane is the area where the collective consciousnesses of all life meld together. It is much like a dream or hallucination. In physical appearance, it is like an endless sea, silver or gold in color. Beams of light pass through it seemingly at random, extending forever in either direction. The various colors are an extension of the other planes. Creatures seeking passage to other planes can use these beams to access them. Angels and demons alike often use the Astral Plane as a middle ground Creatures don't age or experience physical need on this plane, though they can die. Some creatures are able to astral project, sending their consciousness fully into the Astral Plane and separating it from their physical form. They can similarly travel to other planes through use of the beams. Dying while in an astral form simply sends it back to the physical body.
Overall, travel through the Astral plane is an easier, albeit still somewhat difficult, method of planar travel.

The Ethereal Plane

A foggy reflection of the real world, vision is severely limited in the Ethereal Plane. Most creatures can only see out to 10 meters or so in this plane. This plane is where lost souls go, and normally manifest as ghosts. Creatures on the Ethereal plane can see into the real world as if they are looking through dirty glass. However, on the Material Plane, the Ethereal Plane is invisible unless wuth the aid of magic. In certain places, the two planes exist as one, allowing passage one way, the other, or rarely both.

Heaven

Known as Paradise, Heaven is home to entities of pure good. It is a shining place in the sky, a city atop an impossibly tall mountain. It is the home of angels and good deities. Only sufficiently worthy creatures can scale the mountain and reach the city, while the rest dwell around the lower peaks. Even there, they are utterly content with their life.
Worthy creatures in Heaven are under the effect of the Blessing spell, meaning they can add a d4 to any attack roll, skill check, or saving throw. While in the city, they change this to a d8.

Valhalla

This plane is another relatively good one, although it is not pure good. Valhalla sits "on top of the world", meaning the only thing above it is the infinite expanse of stars, and everything else lies below, including seas that stretch on forever. While pious creatures end up in Heaven, righteous creatures end up in Valhalla. A hero that died in a battle with demonic forces is more likely to end up in Valhalla than Heaven. That said, the primary way to enter Valhalla is to die in battle, no matter the reason or the side you fought on- although the Valkyries that descend to take the deceased to Valhalla prefer the brave and the noble. Once in the hall of the gods, creatures constantly fight in the day to prepare for Ragnarok, described as the final battle of all things before the end of the world. At night, the survivors of the battles retire to the massive mead hall in the center of the plane, then drink and sing until they all collapse and wake the next morning.
Creatures do not truly die in Valhalla. Instead, they are revived at dawn the next day, making battle more of a game than anything else.

The Wildlands

This is a plane of endless nature, nearly untouched by man. It echoes the world in a more primeval state, tens of thousands of years before industrialization. As such, the few humans that occupy this plane are primitive in technology, wielding spears, clubs, atlatls, and bows, and subsisting off the land. More advanced ones have access to agricultural technology and tamed animals, but the level of technology in the land never seems to progress any further, despite abundant resources. More advanced humans that live in this region feel a yearning to connect with the land around them as soon as they set foot in this plane. Visitors to this plane have advantage on Survival checks.
If a weapon of a higher technological level is used to kill an animal native to the Wildlands, the user must make a DC10 IQ saving throw or be transformed into that animal, retaining its sapience and speaking ability. At the end of each long rest, the creature can repeat the save. After three failures, the creature is permanently transformed and confined to the Wildlands, forever to roam.

Acheron

If Valhalla is where righteous warriors go, Acheron is where bloodthirsty and evil ones are sent. Those picked over by the Valkyries are sent here automatically, where they are forced to wage war over and over again, for reasons unknown. This plane encompasses the suffering in war, and the very rules of the plane encourage that suffering to continue. Forces clash eternally on grim battlefields, and death is only a brief respite from another grueling session of combat. Those who prove themselves on the battlefield sometimes rise as leaders, commanding over forces on the field of battle.
Creatures do not truly die in Acheron. Once the ongoing battle is resolved, they are revived and forced to fight again. The plane of Acheron itself and the deities that reside on it imbue their fighters with bloodlust. When on Acheron, if a creature kills a hostile one, it gains an amount of HP equal to how much damage the killing blow did.

Hell

A land of hierarchy, and yet complete chaos, Hell is an impossibly deep pit, where the higher levels are subject to rule from the lower ones. It is home to demons and evil deities. Despite the structured appearance, there is no true hierarchy in Hell except raw power and the size of one's loyal forces- but even the most loyal are suspect to become turncoats if there is a greater prize involved. There is no single leader of the demons, although the most powerful are the Barons and Baronesses. These demons stand atop the others only because they command enough power to slaughter any and all challengers to their power. Sometimes a subordinate rises to match their lord's strength and a struggle for control erupts, each demon amassing an army of untold numbers to wage what seems like eternal war against their rivals. Non-demonic creatures on this plane are either immediately accosted or treated like royalty, depending on how weak they appear and act.
In the very bottom is the Infernal Citadel, a sprawling city guarded by the demon elite. Those who wich to converse with the Barons and Baronesses of Hell must travel through all its layers to reach the Citadel.
Sufficiently evil and heartless creatures in Hell are under the effect of the Dark Blessing spell, meaning they can add a d4 to any attack roll, skill check, or saving throw. While in the citadel, they change this to a d8.