One aspect, in literature and lore, does not appear in the
D&D® demon’s powers. That is the ability of a demon to take charge of the
body and mind of lesser mortals.
This ability is known as Demonic
Possession and can really liven
things up for a party of Lawfuls
who are getting along in a dungeon.

Means of Possessing
Only demons of level IV or
greater and the Princes are capable
of possessing bodies of adventurers.
The rule of the thumb is that
only those demons that are immune
to all but magical weapons and
therefore exist upon two planes at
once may possess. The corporeal
body of the demon is sent to
another plane while the spiritual
essence remains in a room, cave,
dungeon hall or other area. The
amulet of the demon must remain on the plane and world in which the
possession takes place. It need not be in the same room but must remain
within one mile of the place of possession. Although the demon should be
placed by the DM, the member of the party affected should be at
random (First person into the room, last person, fourth person or
whatever) so that any member of the party may be the victim and to 
preserve fair play.

Once the victim has been decided,
all members of the party to
enter the room are told that they
feel an oppressive presence in the
room and are asked to roll the dice
for a saving throw. If upon his turn
the victim does not save versus
spells he is possessed. Paladins and
Clerics get a plus one on saving
throw and Princes cause a minus
two. If the save is made no one in the
party is possessed. Only one demon
will inhabit a body at one time and
only one member will be possessed
in one party at one time.

Effects of Possession
The possessed member of the party has the same armor class as the
victim before possession. They may use any weapon except those aligned
Lawful, regardless of previous character class. They attack using these
weapons as whatever level demon they are rather than the level of the
victim. The demon, while in possession, retains all of his powers except for
the ability to gate. Naturally, the demon may not do things that are
precluded by his new shape; no physical flight without wings or no Balrog
immolation. All hits and damage is taken by the infested body rather than
the demon itself.

Upon being possessed, the person will either at once fight to the death or
continue with the party, feeling no effect until the possessing demon
chooses to take control. In any event, the possessed person will register on
Detect Evil or will react violently to Holy Water (Except for a few extreme
cases of females there is little reaction to the presence of the Cross).
Paladins with their power to detect will recognize possession immediately.
If detected the possessing demon will attack. One of the first actions of the
victims that may give away their presence is that they divest themselves of
all aligned weapons or equipment. The victims will lose no points to
alignment shock but will feel intense pain in their presence.

Restraint of the demon possessed individual is almost impossible. They
are able to break all rope and leather restraints and all but the strongest
chains. They may be subdued only by two or more fighters with a strength
of at least 18 (01). Demon princes may not be subdued.

If and when a party member chooses to fight after being possessed there
is a good chance that he will discard treasure, shield, helm, weapons not in
use, and any encumbrance not directly involved with inflicting bodily
harm upon other members of the party. Spells can be cast in order to
subdue the possessed person but they have the demonic spell resistance
and so may not be affected. The actions of a person while possessed are
controlled by the demon so no chaotic action will be held against them.

At the death of the host body the demon may attempt (20% Chance) to
possess the person who struck the killing blow unless they were a Lawful
cleric or a paladin. The save for this is as versus death ray because the victim
is aware of the demon’s presence. If the demon does not choose to possess
or if the saving throw is made the demon will either: stay in his spiritual
form and not attack (35%), transport his spirit to the plane where his body
is and stay there (35%), or tansport his corporeal body to the room he is in
and continue the fight in earnest (30%). If the victim is raised after this
death there is a slight (10%) chance of the demon repossessing the body
after the applicable save versus death ray is attempted.

Exorcism
Every five rounds after two rounds of preparation (no fighting) a cleric
may attempt to exorcise the demon. The ability to exorcise is in direct
proportion to their ability to turn undead. If turn-away is indicated, the
demon is expelled and reacts as if the body was killed except for further
attempts as possession. If dispel is indicated, the demon is expelled and
banished to the plane upon which his corporeal body rests. Regular
demons are rolled for as wraiths. Demon princes are rolled for as for a
spectre. If the demon is driven from the body in this manner there can be no
repossession.

If the amulet of the prince or demon is found he may be ordered to return
to his body on whatever plane it is lodged and leave the victim unhurt. This
may be done and the amulet discarded without any of the attendant
dangers of using a demon’s amulet.
Demons cannot possess a person
who is in control of his amulet and if
a person once possessed comes in 
contact with the amulet of the 
demon possessing him if it causes 
instant death.

Remove curse or limited wish
can occasionally work an exorcism
but the demon gets a saving throw
as a thirteenth level cleric versus
death ray. A full wish is more
effective with the demon getting a
save as a ninth level cleric versus
dragon breath. Princes get a plus
two on these saves.

Effects of Exorcisms
Exorcism is a painful process for cleric and victim as well. Besides not
being able to attempt exorcism without two rounds of not fighting, each
attempt at exorcism costs the cleric 1-6 hit points. The victim reacts in one
of three ways, he can be freed and take 1 - 10 point of damage (55%—, he can
go insane and be treated in the usual ways (25%), or he can drop dead 
(20%); if death results revival may
be attempted.  If a demon is 
exorcised by use of his amulet and 
the command is phrased correctly 
there will be no damage to the 
victim from the exorcism.

Closing Comments
There is no treasure in the room
in which the possession takes place.
The wealth being deposited upon
the plane with the corporeal body.
The demon possessing a human is
not looking for a home. He usually
feels that the body he has taken over
is weak and ugly. The only motive
of a possessing demons to cause
pain and trouble to the victim and
the party with small risk.

Always pay the bills when exorcised or the finance company will
repossess you.