Shambling Cacti

Piercing Absorption. Whenever the shambling mound is subjected to piercing damage, it takes no damage and regains a number of hit points equal to the piercing damage dealt. When stabbed or pierced it grows extra spines and its armour class goes up by 1. It leaks poison (1d8 + 1 = 1 per litre) when its 'skin' is punctured or broken

Multiattack. The shambling mound makes two slam attacks. If both attacks hit a Medium or smaller target, the target is grappled (escape DC 14), and the shambling mound uses its Entangle on it.

Slam. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (4d8 + 4) piercing damage.

Entangle. The shambling cacti entangles a Medium or smaller creature grappled by it. The entangled target is blinded, restrained, and unable to attack, and it must succeed on a DC 15 Constitution saving throw at the start of each of the cacti's turns or take 13 (4d8 + 4) Piercing damage. If the cacti moves, the entangled target is dragged with it. The cacti can have only one creature engulfed at a time per arm.

If an "arm" is cut of it turns into a tiny shambling cacti.