Corporeal Lost Soul Working Draft
Here's a rough draft. I'm giving the basic model 2 claw attacks even though the stats say it has 4, since the description says "A lost soul fights with two claw attacks". It also says the weakest versions have 2 Hit Dice, not 4.
Corporeal Lost Soul
Medium Undead
Hit Dice: 2d12+2 (15 hp)
Initiative: +0
Speed: 30 ft. (6 squares), climb 20 ft.
Armor Class: 13 (+3 natural), touch 10, flat-footed 13
Base Attack/Grapple: +1/+3
Attack: Claw +3 melee (1d4+2)
Full Attack: 2 claws +3 melee (1d4+2)
Space/Reach: 5 ft./5 ft.
Special Attacks: Fear, gnaw, improved grab
Special Qualities: Absorb corpse, all-around vision, damage reduction 5/slashing?, darkvision 60 ft., fast healing 2, unholy toughness, undead traits
Saves: Fort +0, Ref +0, Will +3
Abilities: Str 15, Dex 10, Con —, Int 5, Wis 10, Cha 12
Skills: ---
Feats: ?
Environment: ?
Organization: Solitary?
Challenge Rating: x
Treasure: None
Alignment: Always neutral evil
Advancement: 3-5 HD (Medium); 6-8 HD (Large)
Level Adjustment: —
Fear (Su): ?
Gnaw (Ex): When a lost soul makes a full attack it can make an additional bite attack against any opponent it is grappling (+3 melee, 1d6+1 damage), in addition to its normal claw attacks. It can bite as many opponents as it can grapple. It can also bite a single grappled foe with a standard attack.
Improved Grab (Ex): To use this ability, a lost soul must hit an opponent of any size with a claw attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can gnaw. A lost soul can grapple as many opponents as it has claw attacks at the same time.
Absorb Corpse: If a lost soul kills a humanoid or monstrous humanoid it merges the body with its own flesh, for every victim it absorbs in this fashion the lost soul gains 1 HD, an additional claw attack, and its fast healing increases by 1 point. The victim cannot be returned to life with a raise dead or resurrection spell without first destroying the Lost Soul it has become part of, although it can be brought back to life with a wish or true resurrection (this removes the HD, attack and fast healing advancement the absorbed victim gave the lost soul).
Furthermore, two or more lost souls can merge together into a single lost soul, the Hit Dice, number of attacks and fast healing of the resulting monster equals the sum of the individual lost souls'.
Unholy Toughness (Ex): A lost soul gains bonus hit points equal to its Charisma modifier times its Hit Dice.