Crawler

Decayed. The crawler is always subject to the prone condition.

Destroy the Brain. Whenever the crawler suffers a critical hit, its brain is destroyed and it's hit points are reduced to 0, causing it to die immediately.

Undead Fortitude. If damage reduces the crawler to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is fire or radiant. On a success, the crawler drops to 1 hit point instead.

Grab. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 3) bludgeoning damage, and the target is grappled (escape DC 13). 

Infectious Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target that is surprised or grappled by the crawler. Hit: 3 piercing damage, and the target is infected. At the beginning of each hour for duration that the target is infected, it must make a DC 13 Constitution saving throw or suffer one level of exhaustion. The only way to end the infection is to amputate the bitten area before the infection spreads.

When this attack hits, roll a d20 to determine where the target is bitten and how soon amputation must occur to prevent the infection from spreading. On a 1-4, the target is bitten on a hand, and has 3 hours before the infection spreads. On a 5-9, the target is bitten on an arm, and has 2 hours before the infection spreads. On an 11-13, the target is bitten on a foot, and has 4 hours before the infection spreads. On a 14-17, the target is bitten on a leg, and has 3 hours before the infection spreads. On an 18-19, the target is bitten on the torso, and has 1 hour before the infection spreads. On a 20, the target is bitten on the neck or head, and amputation of the infected area cannot prevent the infection from spreading.

If a humanoid dies from this attack or while infected, a new walker rises from the corpse 2d8 minutes later.