Planar Summonings

Neither celestial nor fiends are readily able to enter the World on their own recognisance. Rarely, a fiend will find a way into the material realm through a flaw in its fabric, but more often they do so in answer to a mortal's summons, and will do all they can to extend their presence. Celestials may also be summoned, or can be dispatched as a deity's emissary.
Devils use pacts and obligations, both to spread their influence and to secure a foothold in the World. Many power­ful pacts require the mortal party to issue repeated summons or create either a physical vessel or portal to allow the devil greater access to the World. Devils will use such access to spread their networks of pacts, and to destroy demons at work on the material planes.
Demons have no such subtle ways of securing their access to the world. Instead, they use force to open paths once given a foothold. Blood shed by or in the name of the Pits can be used to fuel a rift in reality, drawing up the Colossal power of the Pit itself. These rifts not only allow direct access to the World, they also release the corruption of the Pit across a wide area.
Daimons gain access to the world in exchange for services, as everything is transactional to them. Where devils seek control and demons destruction, daimons only desire access to the world to plunder its various forms of wealth. Celestials rarely seek to extend their own time in the world, instead working to enhance the reputation of their god in the brief time allotted to them. The exceptions are the vanishingly rare rogue celestials, usually either higher order celestials who somehow conceive a desire for survival, or forgotten celestials who are never recalled at the end of their assignments.