Some assorted demons, by which I mean (almost) any horrible fiend from Hell. One thing I've found with combining fiends from different sources is that you end up with lots of little inconsistencies. Why should a 13HD ice demon get aura of fear, when a 16HD doomguard (Warcraft MM) doesn't? Why should the doomguard have a lower CR than a 15HD cornugon? Beats me, so I fixed it.
Ice demon: CR 13; Large Outsider (evil, extraplanar, spirit); HD 14d8+84 (154 hp); Init +5; Spd 40 ft, fly 70 ft (good); AC 32 (touch 14, flat-footed 27); BAB +14; Grap +24; Atk +20/+15/+10 melee (2d6+9/x3 plus slow, spear) and +14 melee (2d6+3, bite), or +19/+19 melee (1d10+6, 2 claws) and +14 melee (2d6+3, bite) and +14 melee (3d+3 plus slow, tail); S/R 10 ft/10 ft; SA Spell-like abilities, slow, fear aura, summon demon; SQ DR 10/good, SR 25, darkvision 60 ft, see in darkness, resistances, telepathy; AL LE; SV Fort +15, Ref +14, Will +15; Str 23, Dex 21, Con 23, Int 22, Wis 22, Cha 20.
Skills and Feats: Bluff +22, Concentration +23, Listen +25, Spellcraft +23, Spot +25, Alertness, Cleave, Combat Reflexes, Power Attack, Weapon Focus (spear).
SA — Spell-Like Abilities: At will — cone of cold (DC 20), ice storm (DC 19), dimension door (self only), unholy aura (DC 23), ice darts. Caster level 13th.
SA — Fear Aura (Su): 30 ft radius, enemies with 4 or fewer HD panicked for 2d6 rounds, those with 5 to 13 HD shaken for 2d6 rounds. Will negates DC 22. This is a mind-affecting, fear effect.
SA — Slow (Su): On a hit from the ice demon’s tail or spear; as slow spell for 1d6 rounds. Fort negates DC 23.
SA — Summon Demon (Sp): 1/day — 1d6 barbazu warriors; or 50% chance for 2d4 babau warriors; or 20% chance for another ice demon.
SQ — Resistances (Ex): Acid, fire and electricity resistance 10, cold and poison immunity.
Powerup Results (unholy aura): AC 36 (touch 18, flat-footed 31); SV Fort +19, Ref +18, Will +19.
Horned demon: CR 16; Large Outsider (evil, extraplanar, spirit); HD 15d8+105 (180 hp); Init +7; Spd 20 ft, fly 50 ft (average); AC 35 (touch 16, flat-footed 28); BAB +15; Grap +29; Atk +25/+20/+15 melee (2d6+15 plus stun, spiked chain) and +22 melee (2d8+5, bite) and +22 melee (2d8+5 plus wounding, tail), or +24/+24 melee (2d6+10, 2 claws) and +22 melee (2d8+5, bite) and +22 melee (2d8+5 plus wounding, tail); S/R 10 ft/10 ft (20 ft with spiked chain); SA Spell-like abilities, stun, wounding, fear aura, summon demon; SQ DR 10/good and Britannian steel, SR 28, darkvision 60 ft, see in darkness, resistances, telepathy; AL NE; SV Fort +16, Ref +16, Will +15; Str 31, Dex 25, Con 25, Int 14, Wis 18, Cha 22.
Skills and Feats: Bluff +24, Concentration +24, Intimidate +26, Listen +22, Sense Motive +22, Spot +22, Cleave, Combat Expertise, Iron Will, Multiattack, Power Attack, Weapon Focus (spiked chain).
SA — Spell-Like Abilities: At will — detect good, dimension door (self only), dispel good (DC 21), magic circle against good, persistent image (DC 21); 3/day — fireball (DC 19), lightning bolt (DC 19). Caster level 15th.
SA — Stun (Su): On a hit with spiked chain, stunned for 1d4 rounds, Fort negates DC 27.
SA — Wounding (Su): On a hit with tail, 1 point temp Con.
SA — Fear Aura (Su): 30 ft radius, enemies with 4 or fewer HD panicked for 2d6 rounds, those with 5 to 14 HD shaken for 2d6 rounds. Will negates DC 23. This is a mind-affecting, fear effect.
SA — Summon Demon (Sp): 1/day — 50% chance of 1d6 barbazu warriors, or 20% chance of another cornugon.
SQ — Resistances (Ex): Acid, cold and electricity resistance 10, fire and poison immunity.
Powerup Results (magic circle against good): AC 37 (touch 18, flat-footed 30); SV Fort +18, Ref +18, Will +17.
Doomguard: CR 18; Large Outsider (evil, extraplanar, spirit); HD 18d8+126 (216 hp); Init +5; Spd 40 ft, fly 70 ft (average); AC 35 (touch 15, flat-footed 29); BAB +18; Grap +32; Atk +31/+26/+21/+16 melee (2d6+18/15–20 plus 2d6 unholy, +3 unholy falchion), or +27/+27 melee (1d6+10, 2 slams); S/R 10 ft/10 ft; SA Spell-like abilities, pounce, doom stomp, fear aura, summon demon; SQ DR 10/good and Britannian steel, SR 28, darkvision 60 ft, see in darkness, resistances, telepathy; AL LE; SV Fort +18, Ref +17, Will +16; Str 30, Dex 22, Con 25, Int 18, Wis 20, Cha 22.
Skills and Feats: Bluff +23, Concentration +25, Listen +26, Sense Motive +26, Spellcraft +23, Spot +24, Survival +24, Cleave, Combat Expertise, Great Cleave, Improved Critical (falchion), Improved Initiative, Power Attack, Weapon Focus (falchion).
SA — Spell-Like Abilities: At will — cloudkill (DC 21), detect good, detect magic, dimension door (self only), fear (DC 20), greater dispel magic, flame strike (DC 21), telekinesis (DC 21), unholy aura (DC 24), unholy blight (DC 20); 3/day — destruction (DC 23), greater shout (DC 24); 1/day — fire storm (DC 24), wall of fire, power word stun. Caster level 18th.
SA — Doom Stomp (Su): As earth reaver spell, centered on the doomguard, every 1d4 rounds. Reflex partial DC 29. The doomguard is immune to the effects of its doom stomp and those of other doomguards.
SA — Fear Aura (Su): 30 ft radius, enemies with 4 or fewer HD panicked for 2d6 rounds, those with 5 to 17 HD shaken for 2d6 rounds. Will negates DC 25. This is a mind-affecting, fear effect.
SA — Summon Demon (Sp): 1/day — 50% chance of 1d6 barbazu warriors, or 20% chance of another doomguard.
SQ — Resistances (Ex): Acid, cold and electricity resistance 10, fire and poison immunity.
Equipment: +3 unholy falchion.
Powerup Results (unholy aura): AC 39 (touch 19, flat-footed 33); SV Fort +22, Ref +21, Will +20.