Any Chaotic Evil player character may make a pact with a demon
prince. The demon offers to aid the character on occasion in return for
his soul, and the souls of others through sacrifice. The character, when
killed, cannot be resurrected by any means other than a Wish. Even if
a Wish is used. sooner or later the demon prince will discover that he
has been robbed, and will thereupon immediately hunt for and obliterate that character.
To be useful in making a pact, sacrifices must be performed with
brazier and fire, a stone altar (portable altar weighs 500 gp), bejeweled
silver or gold bowls for victims’ blood, and five minutes of time per
creature sacrificed. Chaotic or unintelligent creatures may not be sacrificed, and non-human types (in this case, all but humans, elves. and
dwarves) count only half value as a successful sacrifice.
In the following formula which determines the chance of success for
a call. S = the average number of sacrifices (in hit dice of creatures
killed) per week made by the character in the past four weeks, including the present one. This value can be a maximum of 20 or four times
the level of the character, whichever is higher. (The referee may adjust
this maximum to account for deaths caused in the normal course of
events by the character, if he wishes.) L = the level of the character
calling upon his patron demon. C = the number of calls made by him
in the past four weeks, successful or not, including the present week
and call.

Percent chance of success: ( (2 times L) + S) divided by C
If this value is not at least one, no call can be made. L is multiplied
by three. not two, for Clerics. The player rolls percentile dice. and if the
result is equal to or lower than the percentage chance, the demon
prince heeds the call and sends some sort of aid. though what kind depends greatly on his whim. A failed roll gives no result, but is still
counted against C.
Whether a call succeeds or fails, only one may be made per week —
or else the demon prince will be very angry. To call requires one
melee round without other activity; the call must be made in a loud
voice in alignment language, using the prince’s name.
The form of the answer is determined by dice rolls. The chance that
the prince himself will appear is nil for characters of lower than fifth
level. From fifth to tenth level, the chance rises 3% per level, so that a
tenth-level character has a 18% chance that, if his call succeeds, the
prince himself will come. Thereafter the chance rises 5% per level. but
may never rise higher than 50%.
When the prince himself comes. percentile dice are rolled to determine his reaction in that melee round, to determine at the start of
each round:

PRINCE'S REACTION

-	Prince uninjured	Prince injured less than 50%	Prince injured 50% or more
Stays	01-80	01-85	01-20
Leaves	81-99	86-98	21-90
Attacks all	00	99-00	91-00

In the first case the prince stays and fights on the summoner’s side.
In the second, he loses interest and leaves, though he may be called
upon again the next week as usual. In the third case he attacks all
present, including the summoner’s party and possibly the summoner,
indiscriminately. Of course, if someone in the summoner’s party attacks the prince, whether willingly or not, that person/creature is
treated by the prince as an enemy.
There is an unchanging 30% chance that if the prince himself does
not appear, he will send a lesser demon. whose ‘type number may not
exceed the level of the character calling for aid. Roll chances progressively, beginning with Type VI or the highest permissible type. The
chance is two percent times the level of the caller for each demon type
until the roll succeeds or until only Type I remains, in which case that
is what appears. Only the six standard demons are rolled for. not
succubi or others. A demon sent in this manner will not gate in more
demons.
If the prince does not appear and does not send a lesser demon, he
will send a monster (Chaotic Evil only) via a Monster Summoning
spell of Level I-VI. Roll d6 to determine which level of spell is used,
then roll percentile dice to generate a specific monster from the proper
list. Note that Level I rolls will always produce a group of manes, or
sub-demons.

MONSTER SENT BY PRINCE

Level 1
01-00 = Demon, manes

Level II
01-44 = Gnoll
45-00 = Troglodyte

Level III
01-38 = Bugbear
39-72 = Ghoul
73-00 = Ogre

Level IV
01-20 = Gargoyle
21-50 = Ghast
51-72 = Lycanthrope, werewolf
73-00 = Shadow

Level V
01-32 = Leucrotta
33-65 = Minotaur
66-00 = Spider, giant

Level VI
01-31 = Jackalwere
32-62 = Salamander
63-00 = Troll

Whatever the form of reply (lesser demon or monster) a delay of 2-
12 minus 7 melee rounds ensues before aid arrives (negative number
equals zero). The referee may prefer to roll dice himself so that the
caller will not know if his call failed or was just delayed. The creature
which comes to aid the character remains for the duration of the
current battle, or if there is no battle. for no less than six consecutive
melee rounds. If a battle begins after its arrival and has not ended at
the end of the 6th round. the summoned creature will remain for the
duration of the battle.
The referee will determine which demon prince becomes a character’s patron. though the player may express a preference. No firstlevel character may have a patron demon. Orcus will not be a patron
for lower than fifth level. Demogorgon not for lower than tenth. No
character may change his allegiance from one prince to another. Remember that non-player characters also may have patron demons,
but only a few have the courage (or foolhardiness) to take the chance.
Obviously. the referee must use great discretion,
Example:  A sixth-level Magic-User has sacrificed 12 captured
elves altogether in the past four weeks. He has not called on his
patron demon in that time. His chance for success when he calls is (2 x
6) + 3 = 15 divided by 1 = 15. Assume he succeeds. The form of
reply is 6% (01-06) the prince, 30% (07-36) a lesser demon. and the
remainder (37-00) a Monster Summoning spell. Say he rolls 24. Now
he rolls percentile dice for the common demon types. from VI down.
hoping for a 12 or lower. If the, first five rolls fail then a Type I demon
automatically comes to his aid. On 2d6 a 6 is rolled, indicating no
delay (6-7 = - 1. effectively 0) in the appearance of aid.