Here's my take on Vil (Oh, and we may have a lot to worry about Vil's Blasphemy effect, since it has a caster level of 40, so everything dies immediately). Assuming we cap Blasphemy at Caster Level 20, so that he only auto-kills level 10 things, his strategy will probably be to charge a dangerous-looking foe while using large amounts of Combat Expertise to keep himself alive and attempt to swallow. Then he will either keep up his Combat Expertise or Power-Attack 30ish Whilrwind Attack, which probably hits all the adventurers thanks to 15-ft reach (and thanks to Combat Brute, this attack will deal about 137 damage), also attempting to swallow all of them. Another possibility is to Spring Attack and resort on his Reach to produce Attacks of Opportunity:
Vildaxaranthus
Gargantuan Outsider
Hit Dice: 50d8+600 (825 hp)
Initiative: +8
Speed: 40 ft. (8 squares) / fly 40 ft. (8 squares) (manoeuvrability Average)
Armor Class: 20 (–4 size, +4 Dex, +10 natural) touch 10, flat-footed 16 (AC 21 vs Dodge target)
Base Attack/Grapple: +37/+71
Attack: Bite +56 melee (4d6+33)
Full Attack: Bite +56 melee (4d6+33) and Tail Slap +50 melee (2d6+22)
Space/Reach: 20 ft./15 ft.
Special Attacks: Improved grab, swallow whole
Special Qualities: Low-light vision, scent, SR 35, Darkvision 60ft, Immunity to Poison, Resistance to acid 10, cold 10, electricity 10, and fire 10, DR 15/epic
Saves: Fort +38, Ref +31, Will +20
Abilities: Str 54, Dex 18, Con 32, Int 13, Wis 15, Cha 14
Skills: Hide +45, Intimidate +55, Knowledge [the planes] +14, Knowledge [nature] +11, Knowledge [local] +11, Knowledge [religion] +11, Knowledge [geography] +6, Knowledge [history] +6, Listen +57, Move Silently +57, Search +54, Sense Motive +55, Spot +57, Survival +55
Feats: Combat Reflexes, Improved Natural Attack (bite), Improved Critical (bite), Power Attack, Improved Initiative, Combat Expertise, Improved Combat Expertise, Dodge, Mobility, Elusive Target, Improved Sunder, Combat Brute, Improved Toughness, Weapon Focus (bite), Iron Will, Spring Attack, Whirlwind Attack
Environment: Forest
Organization: Unique
Challenge Rating: 21
Treasure: None
Alignment: Chaotic Evil
Other special abilities.
Smite Good (Su): Once per day the creature can make a normal melee attack to deal extra damage equal to its HD (maximum of +20) against a good foe
Aura of Tyranny (Su): As a result of his worship by the Falgyr, Vildaxaranthus exudes an aura in a 100-ft radius that suppresses any Freedom of Movement effects or similar abilities unless the effect's caster level is at least 30. Additionally, any living creature that is not an outsider within the radius who does not serve Vildaxaranthus must succeed at a DC 37 Will Save. On a failure, creatures with 4 or less Hit Dice become panicked for as long as they remain with the radius and for 4d6 rounds thereafter, and those with 5 Hit Dice or more become shaken for as long as they remain within the radius and for 4d6 rounds thereafter. A successful save renders a creature immune to the Fear portion of Vildaxaranthus's Aura of Tyranny for 24 hours, but the creature still loses any Freedom of Movement effect it may have, regardless of the save.
Darkness, Poison, Unholy Aura 3/day, Desecrate, Unholy Blight, Contagion, Blasphemy, Horrid Wilting, Summon Monster IX, Unhallow, Destruction, 1/day