The Warp

"The Warp is a strange and terrible place. You might as well throw a traveller into a sea of sharks and tell him to swim home as send him through the Warp unprotected. Better it is not to let common man travel through the stars. Better still, let him not know such a thing is feasible."
-Fra Safrane, 5th Aide to Navigator De'el

Background

The Immaterium, also referred to as the Empyrean, the Aether, the Sea of Souls, the Realm of Chaos, Warpspace or most commonly, "the Warp," is an alternate dimension of purely psychic energy that echoes and underlies the familiar four dimensions of the material universe. The Warp is the source of all psychic powers and known instances of so-called "sorcery" or "magic".
Superficially, the Immaterium is Mankind's solution to the problem of faster-than-light travel. The Warp functions as the medium for FTL interstellar travel, with voidcraft entering it through the use of a Warp-Drive. They then use their own mundane reaction drive systems and the aid of a psychic Navigator to navigate between its flowing currents of psychic energy, as if they were moving through an ocean.
The Immaterium is also rumoured by many cultures, Human and xenos alike, to be the final resting place of the spirits of the dead.

Warp Navigation and the Astronomicon

Before a starship can enter the Warp, it must first travel at sublight speed to a star system's so-called "Mandeville Point." This is the closest distance that a voidship can safely enter or exit the Immaterium from its origin or destination system, respectively.
Once a spacecraft activates its Warp-Drive, it is plunged into a dimension very different from the material universe. It is convenient to imagine Warpspace as consisting of a relatively dense, almost liquid, energy, devoid of stars, light and life as it is commonly known. Once within Warpspace a voidship may move by means of its main Newtonian reaction drive (such as a standard Imperial Plasma Drive), following powerful eddies and currents in the Warp, eventually reaching a point in the Warp corresponding to a destination in realspace.
Navigators are required to navigate the Immaterium with the aid of the great beacon known as the Astronomicon. This blinding light shines through the Warp so that Navigators may bask in the light of The God-Emperor of Mankind and may pinpoint their location in relation to Terra. Most piloted jumps are no more than 5,000 light years at a time, but longer jumps have been made.
Ships must be equipped with a Geller Field in order to survive the currents of the warp, and other even less merciful forces of nature. Should the Geller Field fail, then all would be at the the whims of the masochistic force of Chaos in all its horror.

Warp Channels

In theory it is possible to travel anywhere in the universe through the Warp. However, the shifting tides of the Warp make it easier to travel from some star systems to others, and short Warp jumps are always more accurate than longer ones. This is particularly true when moving a large fleet, which may become spread out across several light years of space over an extended journey.
Long established and well-charted Warp channels connect star systems and entire regions of the galaxy, providing relatively predictable conduits through which the majority of Imperial shipping passes.

The Afterlife

The Ecclesiarchy teaches that when a person passes from this world, if they have devoted themselves and proven devout in the eyes of The God-Emperor of Mankind , their souls will pass into the Warp and ascend to His side, forever protected from the predations of Chaos. Should one fall to Heresy, his soul will fall forever into torment far from the God-Emperor's Light.

The Cicatrix Maledictum

The Great Rift, also known as the Cicatrix Maledictum in High Gothic, is a tear in reality and a raging, galaxy-wide Warp Storm that has essentially rent the territory of the Imperium of Man in half. The rift is known by the other sentient cultures of the galaxy by various names, including the "Crimson Path," the "Mouth of Ruin," the "Warpscar," the "Dathedian," "Gork's Grin" and a thousand other names besides. Even before the fall of Cadia, the Immaterium had been growing ever more turbulent. Travel and communication suffered increasing disturbances as the events of Abaddon the Despoiler's 13th Black Crusade unfolded in 999.M41. Eventually, the Materium could take no more, and a titanic Warp rift tore across the galaxy, spreading from the Eye of Terror to the Hadex Anomaly on the Eastern Fringe. To those on the Terran side of the rift, it is a tainted scar stretching across the sky. To those unfortunates on the far side, it is something much worse -- the very gates of Hell.

Fall of Cadia

The Eye of Terror was the location of a great warp rift in the galactic northwest which predates the Imperium of Man. It is also the location of where all of the heretical forces of Chaos fled following their defeat in the Horus Heresy. Over the millennia, Abaddon the Despoiler led his Black Crusades against the Imperium 13 times. It was during the 13th and latest Black Crusade that Abaddon would turn his attention to the great guardian of the Eye of Terror, the Cadian Gate. The Cadian Gate was the most heavily defended system in all of the Imperium, lying at the very doorstep of the Eye and largely preventing Chaos incursions into Imperial Space. After months and titanic battles by some of the most elite forces of the Imperium, Abaddon the Despoiler crashed a might star fortress into the planet Cadia, cracking its surface and destroying resistance.
With this blow, the Cicatrix Maledictum extended from the Eye of Terror to cut the galaxy in half and the forces of Chaos flow forth to attack the faithful.

Type

Metaphysical, Psychic