Originally Posted by Androxine Vortex
When daemons materialize into our universe, how long can they stay there? (After a time, do they just phase back intot eh warp?)
Someone else may be able to give a more detailed answer.
You pretty much said what needed to be said Serpion
, but to add to your point:
In order for a Daemon to break through into the mortal universe there must be a weakening of the barriers between Warpspace and Realspace (a Warp Rift). These Warp Rifts can occur randomly/naturally or they can be manufactured by a Chaos Power or knowledgeable mortal on either side. These Warp Rifts vary tremendously, from a slight thinning in the dimensional wall that may allow a single Daemon to extend some of its power and influence into the mortal realm to commune with or possess an individual, to vast tears in reality that entire, limitless hosts of Daemons may pour through. Of such value these rifts are, that entire wars are fought for control and possession of them in the warp.
As I said above, Warp Rifts either occur naturally/randomly, or are manufactured by a Chaos Power or a mortal. But there are other circumstances which can bring one about unintentionally; Warp drive implosions is a good example, uncontrolled and rogue psykers can unintentionally bring about rifts, as can cataclysmic wars (which spread death on a massive scale) which disturb the ebb and flow of the warp itself.
To further clarify how a warp rift can be opened by a Chaos God: it doesn't take a lot of energy at all for a Chaos God to open a small rift to allow a single or small number of daemons through into realspace, but a Chaos God will only rarely architect a rift big enough to engineer a full-scale daemonic incursion (which in itself can leave entire sectors devastated) because it may leave him vulnerable to the power and influence of his rival gods, and in the process may be at a disadvantage in the Great Game (by expending such amounts of energy).
As for how long daemonic incursions last after a warp rift has been opened, it varies. Some rifts which vomit forth entire legions of daemons may only last mere moments, the daemons becoming immediately trapped in the material realm and who will quickly succumb to the constant leeching of chaos energy as the rift closes. But essentially it is random and unpredictable, rifts may last seconds, days, years or even centuries. Some warp rifts have endured so long that they can be considered permanent, their energies now self-sustaining (Eg. The Eye of Terror and Maelstrom).
Daemonic invasions are all but impossible to stop by conventional means, the very act of warring against them feeds their own power, and considering large rifts mean they are limitless in number ensures its impossible to defeat them. Only by the closing of the warp rift can a daemonic incursion be defeated, and for the most part there is no way to do this except to let it run its natural course. Once the rift begins to close daemonic legions are at their most violent and dangerous, sensing that their time in realspace is almost up they enact even greater acts of genocide and chaos, and the defenders will ultimately suffer much more before the rift finally seals.
So in answer to the question, it varies greatly. Depending on the strength and way in which the warp rift was brought about, it could be hours or days or it could be centuries, or in the most rarest of cases it could be forever.