This uses the Greater Pseudonatural Template from Dicefreaks, which is like the template given in the ELH, except with some tweaks to things like SR and the use of psi abilities. I've modified the template further by making the tentacle attacks work in addition to the normal attacks, instead of substituting.
Genderless Greater Pseudonatural Lavawight
Chaotic Evil Medium Undead (Fire)
Init: +17; Senses: Listen +41, Spot +41; Darkvision 60 ft.
Aura: Heat (10 ft; 2d10 fire damage; for fire subtype creatures: turn resistance +6, fast heal 10)
AC: 77 (+13 dex, +40 natural, +14 insight), touch 37, flat-footed 64
Hit Dice: 32d12 (384 hp); DR: 10/-; Fast Heal: 10
Resist: Turn +6, Acid and Electricity 30, Fire 150, trait based resistances (see details); SR: 45
Fort +12, Ref +18, Will +24
Speed: 120 ft. (24 squares), Swim 120 ft.
Melee: 2 claws +60 (3d8+27/19-20 (+1d6 on critical hit) plus blazefire), 4 tentacles +60 (2d8+27 plus blazefire), 1 skull butt +52 (2d6+13 plus blazefire)
Special Attacks: Rend 6d8+40, disturbing form, improved grab (tentacles), rotting constriction
Base Attack: +16; Grapple: +37
Space: 5 ft.; Reach 5 ft.
Spell-like Abilities (CL 22) (Save DC 24 + Spell Level):
- At will - dimension door, flame arrow, spell immunity (sleet storm), fireball, wall of fire.
Psi-like Abilities (ML 42):
- At will: concealing amorpha, detect psionics, dimension door (psionic).
Abilities: Str 64, Dex 37, Con -, Int 10, Wis 22, Cha 38
Special Qualities: Undead traits, liquid traits
Feats: Cleave, Great Cleave, Great Fortitude, Improved Critical (claw), Improved Initiative, Power Attack, Weapon Focus (claw)
Epic Feats: Blinding Speed (x2), Epic Weapon Focus (claw), Overwhelming Critical (claw)
Skills: Jump +33, Knowledge (religion) +33, Search +35
Rend (Ex): If the lavawight hits with both of its claws, it latches onto the opponent’s body and tears the flesh. This attack automatically deals an additional 6d8+40 points of Damage.
Blazefire (Su): Living creatures taking Damage from a lavawight’s attack find themselves ignited with blaze-fire; they must succeed at a Fortitude save (DC 40) or permanently lose 4 hit points. The DC is Charisma-based. The opponent must continue to save every round for the next 6 rounds (7 rounds total) to avoid being permanently drained of 4 more hit points each round. The lavawight heals the same amount of Damage whenever a creature is drained of hit points, gaining any excess as temporary hit points. These temporary hit points last a maximum of 1 hour. If an opponent is slain by blazefire, only blackened ash remains of the victim. Hit points lost to the blazefire never heal naturally and cannot be magically restored - they are gone for good.
Undead Traits: Undead recieve +20 to their saving throws against: poison, sleep, paralysis, stunning, disease, death, effects, necromantic effects, mind-affecting effects, and any effect requiring a Fortitude save unless it also works on objects.
All subdual damage, ability damage, ability drain, and energy drain is reduced by 20 (minimum 0). Negative energy heals, but recieving twice the creature's maximum hitpoints in negative energy in a single attack destroys it. Not at risk of death from massive damage, but destroyed at 0 hit points or less. Darkvision 60 ft. Cannot be raised; resurrection works only if creature is willing.
Liquid Traits: The critical multiplier on confirmed critical hits against a liquid creature is reduced by 4, to a minimum of 1 (no multiplier). Sneak attacks against such creatures deal 8 less dice of sneak attack damage.
Heat Aura (Su): A 10-foot-radius spread heat aura surrounds a lavawight. All creatures of the fire subtype in the field (including the lavawight) are treated as if having turn resistance +6 (if undead) and fast healing 10. Creatures subject to fire Damage take 2d10 points of fire Damage each round they remain within the heat aura.
Disturbing Form (Su): A Greater Pseudonatural’s physical form is disruptive to mortal minds, and it’s visage cannot be borne by them for long without mental damage. As a result, any being that is not from the upper dimensions that looks directly at a Greater Pseudonatural creature must make a Will save (DC 33) or else become confused for 32 rounds.
Improved Grab (Ex): If the creature hits an opponent smaller than it with a tentacle, it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity.
Rotting Constriction (Ex): Once the creature has hold of an opponent, each successful grapple check it makes during subsequent rounds permanently drains 2d4 points of Constitution. At the same time, the creature heals 10 hit points, gaining any excess as temporary hp.