Carcass Crab

False Appearance. While the carcass crab remains motionless, it is indistuinguishable from a mound of corpses of discarded arms and armor.

Spiny Defense. A carcass crab’s shell is covered with sharp spines that protrude from between attached metal and bodies. Combined with spears and swords affixed to its shell, these spines make attacking the creature a hazardous proposition. A creature that touches the carcass crab or hits it with a melee attack while within 5 ft. of it must succeed on a DC 14 Dexterity saving throw or takes 5 (1d10) piercing damage and be grappled (DC 15 to escape). A creature grappled this way will take 5 (1d10) piercing damage at the start of it's turn.

Multiattack. The carcass crab makes two claw attacks and pulls one creature it has grappled with it's claw into it's Spiny Defense. 

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 19 (2d8 + 6) bludgeoning damage, and the target must make a DC 14 Dexterity saving throw or be Grappled. A grappled creature is restrained.

Spiny Defense pull. A creature that is grappled by one of it's claws will be pulled into the Spiny Defense by the carcass crab. It must succeed on a DC 14 Strength saving throw or take 5 (1d10) piercing damage and be grappled (DC 15 to escape). A creature grappled this way will take 5 (1d10) piercing damage at the start of it's turn.

Injured rage. The moment the carcass crab is down to half (80) of it's hit points it begins to shake off the corpses, weapons and armor it has collected in a frenzied rage. Every creature within 30 feet of the crab must make a DC 14 Dexterity saving throw or take 4d6 damage from the flying debris and be knocked prone (save halves damage and the creature does not fall prone).

Any creature grabbed in the Spiny Defense at the time of this reaction are thrown off 15 feet and take 2d6 damage (no save).

All squares hit by the rage become difficult terrain from the debris being thrown around.