Crawler

Ambush. The zombie has advantage on attack rolls against a creature that hasn’t taken a turn yet this combat. 

Corpse Camouflage. While the zombie remains motionless, it is indistinguishable from a corpse. 

Destroy the Brain. If the zombie takes a critical hit or is reduced to 0 hit points, it’s brain is destroyed and it immediately dies. 

Undead Fortitude. If bludgeoning, piercing, or slashing damage would reduce the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken. On a success, the zombie drops to 1 hit point instead.

Grab. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) bludgeoning damage, and the target is grappled (escape DC 11). 

Infectious Bite. Melee Weapon Attack: +3 to hit, reach 5 ft., one target grappled by the zombie. Hit: 1 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 11 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 5-10, 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 1d10 minutes later.

Reactive Snap. Whenever a creature comes within 5 feet of the zombie for the first time each turn, if the zombie isn’t grappling a creature, it can make an Infectious Bite attack against that target.