Shambler

Shamblers look like what they are: corpses that forgot to stop moving. The skin has gone sallow and grayish, settling into tones of green, brown, and black where decomposition has advanced furthest. Whatever they wore in life still clings to them in tatters, weathered and rotting along with the body beneath. Limbs hang limp at their sides when idle, swinging loosely with each shuffling step as though the tendons have half surrendered.
  Up close, the details get worse. The flesh has a waxy, sunken quality. Joints bend at angles that suggest the connective tissue is barely holding. The eyes, if they still have them, are clouded and unfocused. There is no recognition in that stare, no spark of the person who used to live behind it. Just the dull, persistent forward momentum of something driven by Infection and nothing else.

Shamblers shrug off ranged weapons with unnerving ease. Bullets, bolts, and thrown weapons lose most of their effectiveness against the rotting tissue; the damage simply does not translate the way it should. Melee weapons remain fully effective, which means the safest way to deal with a Shambler is, paradoxically, the one that puts you closest to its teeth.
  They rise after being put down. Repeatedly. A Shambler that collapses will drag itself back to its feet with a full reserve of stubborn endurance, and the only way to stop the cycle permanently is a Double Tap delivered before it stands. At higher threat levels, they become sturdier: harder to knock around, harder to cripple, and capable of pinning a downed survivor to the ground while they feed. The upgraded variants hit harder and resist attempts to push them back, turning what should be a manageable fight into a grinding war of attrition.

The name says it all. Shamblers move with a slow, stiff gait that looks painful, each step an awkward lurch forward as though the body is fighting its own decay just to stay upright. They never run. They never sprint. The fastest you will see one move is a brisk, reaching stumble when prey gets close enough to excite whatever passes for instinct in a dead brain.
  When they sense the living (and they always seem to sense the living, eventually) their arms come up, fingers grasping at empty air, and they push forward with a kind of mindless desperation. There is no coordination between them when they gather in groups, no pack tactics or flanking. They simply converge, each one drawn independently toward the same target, and the result is a closing wall of dead flesh that tightens from every direction. The collective pressure is the strategy, even if no individual Shambler is capable of having one.
  Murder Gardens breaks the usual pattern. In that Harbormarch district, where the local threshold has grown unstable, Shamblers and other simple hunger-driven dead do not drift away from the morgue core the way they would anywhere else. They pool instead. They gather in the streets, courtyards, and choked lots and stay there, as if the district's heart were pulling them inward, and their numbers keep climbing. The threat is not any single corpse but the accumulation: once enough of them have massed in the outer lanes, the sheer weight pushes the front ranks outward. From there it takes only contact, a scavenger straying too close, a survivor rising through the threshold, or simply enough pressure behind the crowd, to turn a slow buildup into a wave that rolls into neighboring blocks faster than anyone expects. The Ossuary Society and the Blacktide Hunters keep the district in check partly by cutting lanes through the dead to relieve that pressure before it bursts.