Qwillion
First Post
this is redundant as you state it gains the outsider sub type which grants it darkvision 60'Darkvision 60 ft.
caster level should be listed "caster level equal to HD"Demons with at least 6 HD gain Greater Teleport at will
only weapons that are part of the base creautures description including natural weapons, it should not pick up the holy avenger and work the same way.Aligned Weapons (Su): weapons used by the demon count as chaotic and evil for the purpose of overcoming damage resistance .
Timeless body (Ex): A demon's body does not age,
or breathe.nor can it die from starvation or thirst. Demons do not need to sleep
the quoted potion should be removed as it is redundent with the outsider creature type.
DAMAGE REDUCTION
Good work and fine research.
How about epic, cold iron and goodNote: Cold iron and good might be better than epic. Considering "downgrading" the DR of a powerful fiend.
I do wonder why lawful never hurts chaotic subtyper creatures

Natural Armor increases to (4 + CR/2 + HD/6); reduce by any insight or deflection bonuses the creature may possess, and remove class derived armor and shield proficiencies. Racial or feat derived proficiencies are retained.
The natual armor bonus seems wiered and unwieldy, though I will admit to not having done the research. Also the way you have it written does not stat base CR or adjusted CR.
why because i am a demon can I no gain my class benifits if I advance in fighter or if I have been wearing it for years can I suddenly no longer wear armor or use a shield?
Ability Scores: The demon's stat array (before adjusting for size) should be around 75+2/HD. Creatures with 15+ HD will have more (75 + 4/HD for a balor or a marilith), and Creatures with HD of 4 or less will have less (63 for the 2 HD dretch). Add +1 CR/LA if more than +10 has to be added. Divide the bonuses fairly evenly.
A very interesting approach
Special abilities: The creature should have 1 spell-like or supernatural ability per 2 HD. Count any existing abilities towards this limit. See the MM and BoVD demon entries for ideas. Adjust final CR if a demon has unusually strong (or weak) abilities.
You should include a notation that no ability should have the good or lawful descriptor. again a very interesting approach though a selection of common abileies could not hurt. Thier should also be a range of uses per day vs. level of spell-like aiblity as compared to HD.
Challenge Rating: current CR+1 or HD, whichever is higher
I understand people don't like this but take a look at the 3.5 fiendish template,