Outsiders are barred from entering the plane because the only two planes connected to the material are the Ethereal and Shadow. The force in which that erected this barrier made so the world itself is toxic to outsiders. Any outsider that manages to pass into the barrier of the world will take 2d6 points of “soul” damage each round they exist in their natural form. An outsider only regains 1 soul hp per week. An outsider’s maximum soul hp is equal to his hit die divided by two times his number of hit dice plus his wisdom modifier per hit dice he has. For example a 5 HD creature with d8 hit dice and a 2 Wisdom modifier would have (4[Half a D8]*5+2*5) in total he would have 30 soul hp.
Schools of magic that summon monsters and creatures that are outsiders fail to work in the usual sense. They summon what the summoner thinks or believes the creature to be. They are summoned through the stagnant residual energy surrounding them, as such there are no alignment restrictions while summoning because the summons are “created” for the purpose and not an actual creature. Eidolons fall under special rules in this world, they aren’t really summoned more created when the summoner manifests them, they are essential part of the summoner’s soul.
Outsider Body Hopping
Since Outsiders cannot enter the plane naturally and when they do their soul starts to be devoured by the force that is actively repelling them. Some outsiders have realized they have the ability to subside in a mortal to alleviate the effects of the world crushing down on them. Each and every outsider type has their own spin on the methods in which they can inhabit a mortal body. But they all have the option regardless of their type to forcibly enter a mortals body and subside within them. What comes after is dependent on an outsiders aligning views, if they malevolent and wicked they will tend to take control over the body and permanently secure the body as its own while the other side will stay in the back and make the mortal forget what happened, so they may coexist in some sort of mutually beneficial harmony.
Outsider “Soul” Stones
Outsiders who occur in this plane without having to subside within a mortal are considered naturally occurring. They’re still outsiders but upon death their form crumbles away and reveals a stone swirling with a certain color/tint of energy. So far no knows where these stones come from or how the outsiders bind their form/soul to them. But it has been witnessed that an outsider can eat one of these stones of their fallen enemy and gain more power. This form of power acquisition is somewhat artificial as it is most certainly an evil act as the soul (even if it is that of an evil outsider) is being consumed and destroyed which regardless of the alignment of the soul being consumed/destroyed is an vile act.