judging by the head adorning his loincloth, I'd say he's a Medium Outsider (Evil)
the real differnce between demons and devils is the alignment
Chaotic Evil = Demons
Lawful Evil = Devils
Does it have any special attacks or qualities . . . Like imunities?
The only demon subtype listed in the Monster Manual are tanar'ri:
Outsider (Chaotic, Extraplanar, Evil, Tanar'ri)
Immunity to electricity and poison
Resistances: acid 10, cold 10, fire 10
Summon (Sp): % chance to summon certain demons
Telepathy
The only devil subtype listed in the Monster Manual are Baatezu:
Outsider (Baatezu, Evil, Extraplanar, Lawful)
Immunity to fire and poison
Resistances: acid 10, cold 10
See in Darkness (Su)
Summon (Sp): % chance to summon certain devils
Telepathy
that's the basic traits.
Because the image shows your BBEG bathed in large flames, it's probably a devil or a demon<(with imporved fire resistance).
looks like it'd have
Barbed Defense (Su): Any creature striking a (insert race here) with handheld weapons or natural weapons takes 1d8+(Str bonus) ponts of piercing and slashing damage from the (demon/devil)'s barbs. Note that weapons with exceptional reach, such as longspears, do not endanger their users in this way.
and
Powerful Build (Ex): The physical stature of a (insert race here) lets it function in many ways as if he were one size category larger. Whenever a (insert race here) is subject to a size modifier or special size modifier for an opposed check (such as during grapple checks, bull rush attempts, and trip attempts), the (insert race here) is treated as one size larger if it is advantageous to it. A (insert race here) is also considered to be one size larger when determining whether a creature's special attacks based on size (such as improved grab or swallow whole) can affect it. A (insert race here) can use weapons designed for a creature one size larger without penalty. However, its space and reach remain those of a creature of its actual size. The benefits of this characteristic stack with the effects of powers, abilities, and spells that change the subject's size category.
it might also have
Infernal Wound (Su): The damage a (insert race here) deals with its (insert name of weapon here) causes a persistent wound. An injured creature loses X additional hit points each round. The wound does not heal naturally and resists healing spells. The continuing hit point loss can be stopped by a DC 13 + (Con bonus) Heal check, a cure spell, or a heal spell. However, a character attempting to cast a cure spell or a heal spell on a creature damaged by a (insert race here)'s (insert name of weapon here) must succeed on a DC 13 + (Con bonus) caster level check, or the spell has no effect on the injured character. A successful Heal check automatically stops the continuing hit point loss as well as restoring hit points. The infernal wound is a supernatural ability of the (insert race here), not of the weapon. The check DC is Constitution-based.
That might also be a Large Morningstar in it's hand.
We're also looking at DR 5, 10, or 15 with the exception of good-aligned weapons or affects.
