It is unknown whether Death's Doors were created by Sorcerers and Necromancers, or by the infernal powers themselves to tempt mortals to their doom. No two Death's Doors look the same, and some are sufficiently non-descript that there is no outward clues to their true identity and function.
A Death's Door, when placed into an doorway, functions as a passage to the Underworld. The Underworld to which Death's Door opens depends on the user, so the Death's Door could open to different infernal planes for each person attempting ingress.
Death's Doors can weigh several tonnes, but because they are magical, regardless of the weight of the door they can be carried by two or more people, or transported in a cart or wagon.
The standard use of a Death's Door is to attempt the recovery of a fallen comrade or lover from the afterlife. The term "hovering at Death's Door" refers to the hesitance of the users to pass through Death's Door, for several reasons. First, they must brave the dangers and horrors of the underworld. Second, once the comrade or lover is located, every demon, devil or other denizen within range will be alerted to a mortal's presence, and will make every effort to capture the interloper(s). Third, there is a chance that the deceased will refuse to accompany the searcher back to the land of the living, as they are quite content where they are or feel they have no reason to return to a mortal life.
Once placed in a doorway, a Death's Door cannot be removed by infernal creatures. That, along with the fact that the users of the Door will unerringly know in which direction the Death's Door is located, provides some assurance that once an individual uses the door, they will be able to find their way back. However, non-infernals can move the door, and if that is done, knowledge of the Door's location is lost, and the person(s) previously using the door may be trapped in the underworld, unless the can find another Death's Door location or other passage to the land of the living.
To operate the Death's Door, the user must think of the deceased person, and recite an incantation that asks the Death's Door to find and allow passage to the person being so located. The Death's Door will typically open a passage to the underworld that is within one mile of the deceased, although, being a somewhat capricious form of magic, the door may open above a lava flow, chasm, vacuum, or other deadly location.