Outsiders

In D&D mortals live on the Material Plane, but they might be visited by beings form other planes of existence:
 * Angels are a celestial agents sent forth into the planes to further their god's agenda for weal or woe. Their sublime beauty and presence can drive awestruck onlookers to their knees. Yet angels are destroyers too, and their appearance portends doom as often as it signals hope. Angels are paragons of Good and Law, which can make dealing with them problematic as they never compromise. Also note that neither Good nor Lawful means Merciful. As agents of Law and Good angels are forbidden from lying (but they can choose not to reveal things).
 * Demons are spawned in the Infinite Layers of the Abyss, and are the embodiment of Chaos and Evil, they are engines of destruction barely contained in monstrous form. Possessing no compassion, empathy, or mercy, they exist only to destroy. Demons respect power and power alone and is incredibly hard to coerce them into service. Due to their chaotic nature the word of a demon is essentially worthless.
 * Devils personify tyranny, with a totalitarian society dedicated to the domination of mortal life. The shadow of the Nine Hells of Baator extends far across the multiverse, and Asmodeus, strives to subjugate the cosmos to satisfy his thirst for power. To do so, he must continually expand his infernal, armies, sending his servants to the mortal realm to corrupt the souls from which new devils are spawned. Devils are agents of Law and Evil and often try to strike bargains with mortals, but a mortal making such a bargain must be wary. Devils are crafty negotiators and positively ruthless at enforcing the terms of an agreement. Moreover, a contract with even the lowliest devil is enforced by Asmodeus's will. Any mortal creature that breaks such a contract instantly forfeits its soul, which is spirited away to the Nine Hells. Only divine intervention can release a soul after a devil has claimed it.

Death of Outsiders
In D&D outsiders (e.g. angels, devils, demons) often visit the mortal plane, where they can be defeated just like any other being. However, if they died in a plane that is not one of their home planes then they reform back home after a short amount of time. Because of this it is not at all trivial to deal with a hostile outsider. Fundamentally there are 3 ways to rid yourself of them:
 * 1) Band-aid solution: Kill them on the Material Plane - This is by far the easiest, as outsiders often enter the Material Plane alone, but those slain there will soon return to life, likely with a thirst for vengeance. Still, it might be a worthwhile solution in two cases: a) demons get banished back to the Abyss which is such a violent place that there is a good chance they will get killed there before they can return to bother you again (unless they are very powerful) b) devils are often tasked with important missions, if they fail they often get "demoted", which involves a painful transformation to a lower level devil while (possibly) losing all their memories, thus they won't remember to come back seeking revenge
 * 2) Half measure: Imprisonment - Favored method of wizards, by building a prison to hold the outsider you can essentially pass on the problem to future generations. These prisons can hold for hundreds or even thousands of years, until (eventually) some fool comes and opens them. But by that time the original builders are long dead, so the prison bought (at least for them) peace of mind
 * 3) Full measure: Slaying them in their home - By far the hardest, but it is a permanent solution, once a devil/demon is slain in their home plane, they are gone for good, and shall never return. It is very advisable to prepare well for such an expedition and to find allies in the plane (usually enemies of the target)