(From Mordenkainen's Tome of Foes - 2018):

The ancient war machines known as cadaver collectors lumber aimlessly across the blasted plains of Acheron until they are called upon by a necromancer, hobgoblin general, or other evil warlord to bolster the ranks of a conquering army. These fearsome constructs obey their summoners until being dismissed back to Acheron, but if a summoner comes to a bad end, a cadaver collector might wander the Material Plane for centuries, collecting corpses while searching for a way to return home.

Sweeping the Dead. Cadaver collectors respond to a summons from a mortal only when they are called to the scene of a great battle — either where one is in progress, where one is imminent, or where one once took place. They encase themselves in the armor and weapons of fallen warriors and impale the corpses of those warriors on the lances and other weapons embedded in their salvaged armor.

Conjured Berserkers. Corpses that accumulate on the construct’s shell aren’t just grisly battle trophies. A cadaver collector can summon the spirits of these cadavers to join battle with its enemies and to paralyze more creatures for eventual impalement. Although these specters are individually weak, a cadaver collector can call up an almost endless supply of them, if given time.

Constructed Nature. A cadaver collector doesn’t require air, food, drink, or sleep.

(From 3.5e Monster Manual III - 2004):

This massive, hunched creature built of stone and metal looms over you. Its long, heavily plated arms nearly drag on the ground from shoulders flanking a flat-featured, oblong head, but its back draws your eye. Spikes of various sizes—all barbed and covered in gore dried and fresh—sprout from the plates of metal on its back. A few bodies remain impaled on the spikes, their mouths open in silent screams.

Cadaver collectors were originally built to serve exactly the purpose their name implies. Those with an interest in collecting the dead would deploy the constructs to bring bodies back from a battlefield. Corpses gathered in this fashion are often used by generals to provide information on an enemy's forces, as well as to fuel magical or medical research.

Most often, though, cadaver collectors were dispatched to gather bodies for the creation of undead. This activity after a battle typically resulted in retaliation from either or both sides, as few armies want to see their dead return to walk the land in undeath.  

In times of peace, cadaver collectors usually sit idle, finding out-of-the-way locations near the site of their last great battle to wait for a new master to give them new orders. Others find employment performing their grisly task for masters who are not so discriminating about where corpses are collected. Still others have found, through a certain perverse twist of logic, that if no battle can be found, they can find ways to start a conflict so that bodies become available and they can realize their purpose once more. Some have just ceased to function correctly. With a lack of purpose, they are unable to distinguish between bodies living or dead, and collect each indiscriminately.  

A cadaver collector, even hunched over, stands about 12 feet tall and weighs about 4,000 pounds. They do not speak but understand one language, usually Common, known to their creator.

Combat: A cadaver collector typically begins a battle with its breath weapon, then grabs any paralyzed opponents and impales them on its spikes to die before turning to deal with other foes. If enemies prove resilient to its breath, it doesn’t hesitate to snatch up and impale any enemy it can reach. 

Breath Weapon (Su): 30-foot cone, once every 1d4 rounds, paralyzing gas, Refl ex DC 18 negates. The save DC is Constitution-based.  

Impale (Ex): If a cadaver collector has successfully grappled a Medium or smaller creature, it can attempt to impale the creature on its back spikes by making a successful grapple check. Doing so is a standard action that does not provoke attacks of opportunity. If the collector successfully impales a creature, the creature takes 4d8+15 points of damage and is considered pinned. Each round, the creature takes another 2d8 points of damage as the movement of the cadaver collector causes additional pain to the impaled creature.  

The creature cannot break free unless it makes a DC 28 Strength check. Success indicates the creature is free of the spike but takes an additional 2d8 points of damage in the process. Failure means the creature takes 4d8 points of damage and remains pinned in place. An ally can try to free an impaled creature with a DC 20 Strength check.  

Once a cadaver collector has a creature or creatures impaled on its back spikes, it is no longer considered to be in a grapple and can attack other opponents without penalty. It cannot use its slam attacks against impaled opponents. Creatures that get free of a cadaver collector become its immediate and most urgent targets. It typically pursues such creatures exclusively if it has any chance of catching and impaling them again.  

A cadaver collector’s back spikes can hold up to 4 Medium creatures, 16 Small creatures, or 64 Tiny creatures. Creatures smaller than Tiny are too miniscule to be impaled on the collector’s back spikes, although the collector can still deal 4d8+15 points of damage to such opponents by grinding them into its back spikes with a successful grapple check once it has them grappled. 

Improved Grab (Ex): To use this ability, a cadaver collector must hit with a slam attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity. If it wins the grapple check, it establishes a hold and can attempt to impale the following round. 

Trample (Ex): Reflex DC 28 half. The save DC is Strength-based. 

Immunity to Magic (Ex): Like golems, cadaver collectors are especially resilient to magic. However, this resilience takes a slightly different form in the cadaver collector.  

A cadaver collector is immune to any spell or spell-like ability that allows spell resistance. In addition, spells of 3rd level or lower that target a cadaver collector automatically rebound back at their caster as if the collector were under the effects of an unbeatable spell turning effect. This effect cannot be dispelled or suppressed, except against spells that deal electricity damage (see below). Spells that deal rust damage or alter stone penetrate the immunity to magic. Other spells and effects function differently against the creature, as noted below.  

A magical attack that deals sonic damage slows a cadaver collector (as the slow spell) for 3 rounds, with no saving throw. Sonic spells of 3rd level or lower are absorbed by the collector and not refl ected.  

A magical attack that deals electricity damage is absorbed rather than refl ected. Such an attack breaks any slow effect on the collector and heals 1d6 points of damage for every 6 points of damage the attack would otherwise deal. If the amount of healing would cause the collector to exceed its full normal hit points, it gains any excess as temporary hit points.  

A cadaver collector is affected normally by rust attacks. In addition, magical attacks that alter the nature of stone, such as transmute rock to mud, deal 1d6 points of damage per spell level to the cadaver collector.

Greater Cadaver Collector:

An impressive construct built more sturdily than a standard cadaver collector, a greater cadaver collector stands about 30 feet tall and weighs about 15 tons. 

Combat: In addition to the tactics common to all cadaver collectors, a greater cadaver collector also chooses to bull rush, overrun, and trample to separate and pulp opponents. 

Breath Weapon (Su): 30-foot cone, once every 1d4 rounds, paralyzing gas, Refl ex DC 27 negates. The save DC is Constitution-based. 

Trample (Ex): Reflex half DC 41. The save DC is Strength-based.

Cadaver Collectors in Eberron: 

Cadaver collectors are found throughout the Mournland, slowly picking their way through battlefields.