Advancement: 13 HD + (Colossal)
Next: Detail the spell-like abilities.
Here is an attempt at the statblock of the beast. I took some liberties (Cold subtype and all-around vision) to round it out. Areas with a ? still need to ne worked out.
Death's Grip Mantis
Colossal Undead (Cold)
Hit Dice: 12d12 (78 hp)
Initiative: -4 (Dex)
Speed: 20 ft.
AC: 23 (-8 size, -4 Dex, +25 natural)
Attacks: 2 Bites +19, 4 claws +14; or touch +19
Damage: Bite 2d6+21, claw 2d4+10; touch 4d12 cold
Face/Reach: 40 ft. by 40 ft./15 ft.
Special Attacks: Freezing grasp, death throws, breath weapon, spell-like abilities, vorpal bite
Special Qualities: Undead, cold subtype*, all-around vision*
Saves: Fort +4, Ref +0 Will +9
Abilities: Str 53, Dex 3, Con \, Int 3 , Wis 12, Cha 6
Skills: Climb + 22, Jump +23, Listen +2, Search +0, Spot +5, Wilderness lore +3.
Feats: Cleave, Great Cleave, Power Attack, Sunder
Climate/Terrain: Any land and underground
Organization: HORDE!!! (20-80)
Challenge Rating: 13
Treasure: Standard coins, 50% items, no goods
Alignment: Always lawful neutral
Advancement: 13 HD + (Colossal)
This ghastly creature is born from villages that starve in cold winters. Somehow, they find each other and go hunting in hordes, swallowing whole cities during their rampage. It resembles a colossal mantis with two front bodies.
Combat
Death's grasp mantii (is that the plural?) are intelligent enough to soften up their targets with their spell-like abilites. They are comparably frail for their size, so they avoid melee until they are certain that the targets do not fight back too strongly. However, due to their ceaseless hunger, they will enter melee sooner or later.
Freezing Grasp: With a mere touch, this creature can cause 4d12 points of cold damage
Death Throes (Ex): If a death's grip mantis is reduced to 0 hit points, it is destroyed, and explodes in a multi-colored blast of light. All creatures within 60 ft. must make a Fortitude save (DC 14) or be blinded for 2d4 rounds.
Those who successfully save against the death throes effect gain a +2 morale bonus to attack and damage rolls made against all surviving death's grip mantises, as well as a +2 bonus against to saving throws against their special attacks, spell-like abilities, and death throes. Increase the bonus by +1 for each additional successful save made against a death throes effect in the same round (to maximum of +5). These bonuses last for one hour.
Breath Weapon (Su): The Death's Grip Mantis has one breath weapon: a cone of blistering cold that wails like the screams of the frozen dead. The cone extends to a distance of 60 feet, and those caught within the cone must make a Reflex Save (DC 16) or suffer 6d8 points of cold damage, as well as come under an insanity effect. Those who succeed at the save take half damage, and do not suffer the insanity effect. The caster level for the insanity is 12th.
Spell-like Abilities: At will - Magic Missile, Mage Armor, True Strike and Spider Climb; 3/day - Meteor Swarm, Energy Drain, Imprisonment, and Time Stop; Caster level 20th. Save DCs are 8 plus spell level.
Vorpal Bite (Ex): On a critical hit with it’s bite, this beast devours the head of it’s opponent, which usually results in the death of the victim.
Cold Subtype: Immune to cold, double damage from fire except on a successful save.
All-Around Vision: Cannot be flanked, +4 racial bonus to Search and Spot checks.
Skills: +4 racial skill bonus to Listen checks.