Great Ice Crab

My PCs have conquered the bad guys in “A Darkness Gathering” and now head off to the North in “Masters of Eternal Night“.  This is one of the possible encounters the PCs may stumble upon as part of the skill challenge.

Inspiration: Dark Souls great crab.

Medium monstrosity, unaligned
Armor class 17 (natural armor)
Hit points 136 (13D12 + 52)
Speed 30 ft., swim 30 ft., burrow 30 ft.

25 (+7) 8 (-1) 19 (+4) 1 (-5) 11 (+0) 3 (-4)

Skills Perception +5, Stealth +3
Skills Stealth +4
Damage Immunities Cold
Senses blindsight 30 ft., passive perception10
Languages —
Challenge 8 (3,900 XP)

Amphibious. The great crab can breathe air and water.
Rummaging Heal. If the great crab has no targets within 30 feet of it, it burrows and moves towards the nearest target. While burrowing it heals 20 hit points or, back to half it’s hit points if it is below half (back to its bloodied state).
Ice walk. The great crab can move across and burrow through icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn’t cost it extra movement.
Shell Camouflage. While the great crab remains motionless with its eyestalks and pincers tucked close to its body, it resembles a natural formation or a pile of snow. A creature within 30 feet of it can discern its true nature with a successful DC 15 Intelligence (Nature) check.
Tough shell. The great crab’s shell has a damage threshold of 10. Attacks that specifically target the underbelly area near its mouth (AC 21) ignore this trait.

Multiattack. The great crab uses frostbite if available, then makes two attacks with each of its claws.

Left Claw. Melee weapon attack: +10 to hit, reach 5 ft., one target.
Hit: 12 (1d10 + 7) bludgeoning damage, and the target is grappled (escape DC 15). A grappled creature automatically takes 22 (3d10 + 7)bludgeoning damage at the start of its turn.

Right Claw.Melee weapon attack: +10 to hit, reach 10 ft., one target.
Hit: 33 (4d12 + 7)bludgeoning damage. The right claw scores a critical hit on a 19 or 20.

Frostbite (Recharge 6). The great crab exhales an icy blast in a 30-foot cone. Each creature in that area takes 45 (10d8) cold damage and is slowed (as the slow spell). At the end of a creatures turn it may attempt a DC 15 Constitution saving throw to end the slow effect. If the creature is already slowed by this attack it takes an extra die of cold damage. This effect is cumulative. If it dies from this attack it becomes an ice sculpture.