Xerfilstyx (CR 15)

Large Outsider (Baatezu, Evil, Extraplanar, and Lawful)
Alignment: Always lawful evil
Initiative: +5(+1 Dex, +4 Improved Initiative); Senses: see in darkness, Listen +17, and Spot +17
Languages: Celestial, Common, Draconic, Infernal; telepathy 100 ft.

AC: 32 (-1 size, +1 Dex, +22 natural), touch 10, flat-footed 31
Hit Dice: 15d8+105 (172 hp); DR: 10/good or silver
Fort +16, Ref +10, Will +8
Speed: 50 ft., Fly 60 ft. (poor), and Swim 50 ft.
Space: 10 ft./10 ft.
Base Attack +15; Grapple +28
Attack: Melee: 2 claws +24 each and bite +22 and tail +21 and gore +21 or Melee: gore +23 with Powerful Charge* (includes +2 attack bonus for charging)
Damage: claws 1d6+9, bite 1d8+4, tail 1d8+4, 1d8+4 (or gore 1d8+13 plus 2d6)
Special Attacks/Actions: breath weapon, summon baatezu
Attack Options: Power Attack, blood drain, improved grab, toss
Abilities: Str 28, Dex 12, Con 25, Int 15, Wis 9, Cha 21
Special Abilities: blood drain, breath weapon, fear aura, improved grab, toss; Spell-like abilities
Special Qualities: Styxborn
Feats: Improved Initiative, Multiattack, Power Attack, Powerful Charge, Weapon Focus (bite), Weapon Focus (claw) ,
Skills: Bluff +23, Concentration +25, Diplomacy +7, Disguise +5 (+7 acting), Hide +15*, Intimidate +7, Knowledge (arcana) +21, Knowledge (the planes) +21, Listen +17, Move Silently +19, Search +21, Spellcraft +23, Spot +17, Survival -1 (+1 on other planes or to follow tracks), and Swim +35
Advancement: 16-30 HD (Large); 31-45 HD (Huge)
Climate/Terrain: River Styx
Organization: Individual, School
Treasure/Possessions: Standard

Source: Fiendish Codex II
Fiend Folio

Spell-Like Abilities (CL 15th): At will - confusion (DC 19), dispel magic, hold person (DC 17), greater teleport (self plus 50 pounds of objects only), wall of ice; 3/day - cone of cold (DC 20), ice storm, legend lore; 1/day - insanity (DC 22), power word blind

Blood Drain (Ex): A xerfilstyx drains blood from a grappled opponent, dealing ld6 points of Constitution damage each round it maintains the hold. This ability does not affect elementals, plants, or creatures that lack a Constitution score.

Breath Weapon (Ex): 50-foot cone, damage 15d8 (Reflex DC 24 half) and 1d6 Intelligence (Will DC 24 negates). Once a xerfilstyx uses this ability, it must deal at least 6 points of Constitution damage with its blood drain before using it again.

Fear Aura (Su): At the end of each of a xerfilstyx's turns, creatures within 20 feet of it must succeed on a DC 22 Will save or be panicked for 10 rounds. The save DC is Charisma-based.

A creature that successfully saves cannot be affected again by that same xerfilstyx's aura for 24 hours. Baatezu are immune to the aura.

Improved Grab (Ex): To use this ability, a xerfilstyx must hit an opponent of any size with a tail attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity.

Styxborn (Ex): Xerfilstyxes are immune to the effects of the River Styx.

Summon Baatezu (Sp): Once per day, a xerfilstyx can attempt to summon another xerfilstyx with a 50% chance of success. This ability is the equivalent of a 7th-level spell (CL 15th).

Toss (Ex): If a xerfilstyx charges and hits a Medium or smaller opponent with one of its attacks, it can make a trip attempt as a free action without provoking attacks of opportunity. If it succeeds in tripping its opponent, it flings the opponent into the air, dropping it 1d6 squares away in a random direction. Creatures so affected take 1d6 points of falling damage.

Skills Xerfilstyxes have a +8 racial bonus on any Swim check made to perform some special action or avoid a hazard. They can always choose to take 10 on Swim checks, even if distracted or endangered.

Updated to Fiendish Codex II

Baatezu Subtype

Many devils belong to the race of evil outsiders known as the baatezu.

Traits: A baatezu possesses the following traits (unless otherwise noted in a creature's entry).

Evil Subtype

A subtype usually applied only to outsiders native to the evil-aligned Outer Planes. Evil outsiders are also called fiends. Most creatures that have this subtype also have evil alignments; however, if their alignments change, they still retain the subtype. Any effect that depends on alignment affects a creature with this subtype as if the creature has an evil alignment, no matter what its alignment actually is. The creature also suffers effects according to its actual alignment. A creature with the evil subtype overcomes damage reduction as if its natural weapons and any weapons it wields were evil-aligned (see Damage Reduction).

Extraplanar Subtype

A subtype applied to any creature when it is on a plane other than its native plane. A creature that travels the planes can gain or lose this subtype as it goes from plane to plane. This book assumes that encounters with creatures take place on the Material Plane, and every creature whose native plane is not the Material Plane has the extraplanar subtype (but would not have when on its home plane). An extraplanar creatures usually has a home plane mentioned in its description. These home planes are taken from the Great Wheel cosmology of the D&D game (see Chapter 5 of the Dungeon Master's Guide). If your campaign uses a different cosmology, you will need to assign different home planes to extraplanar creatures.

Creatures not labeled as extraplanar are natives of the Material Plane, and they gain the extraplanar subtype if they leave the Material Plane. No creature has the extraplanar subtype when it is on a transitive plane; the transitive planes in the D&D cosmology are the Astral Plane, the Ethereal Plane, and the Plane of Shadow.

Lawful Subtype

A subtype usually applied only to outsiders native to the lawful-aligned Outer Planes. Most creatures that have this subtype also have lawful alignments; however, if their alignments change, they still retain the subtype. Any effect that depends on alignment affects a creature with this subtype as if the creature has a lawful alignment, no matter what its alignment actually is. The creature also suffers effects according to its actual alignment. A creature with the lawful subtype overcomes damage reduction as if its natural weapons and any weapons it wields were lawful-aligned (see Damage Reduction).