Lodestone Marauder (5e Creature)

Large aberration, unaligned

Armor Class 24 (natural armor), 26 against metal weapons (see Magnetic Defense below)
Hit Points 126 (11d10 + 66)
Speed 30 ft., burrow 20 ft., climb 20 ft.

Saving Throws Str +7, Dex +5, Con +10, Wis +5
Skills Athletics +7, Perception +5
Proficiency Bonus +4
Senses darkvision 60 ft., passive Perception 15
Languages —
Challenge 9 (5,000 XP)

Magnetic Defense. The lodestone marauder adds +2 to its AC (included in the calculation) against attacks made by weapons made wholly or substantially of metal.

Stability. While on the ground, the lodestone marauder has advantage on Strength and Dexterity saving throws made against effects that would move it or knock it prone.

Many an adventurer delving into the depths of the Underdark have been walking along a tight cavern, only to have their weapons ripped from their hands before they meet their end at the claws and teeth of a lodestone marauder. These monstrous creatures stand on spindly but muscular limbs that terminate in hooked claws, have jaws that can bite through steel like flesh, and their body is covered in jagged spines, rusty from the iron that infuses into their armor.

Magnetic Monster. Lodestone marauders can be found in mountains, especially ranges rich in iron, but mostly they are found in the Underdark where such minerals are abundant. Some however do find themselves drawn to cities and other populated areas, particularly those with strong metalworking industries that they can scavenge off of. These individuals are the most dangerous because they are the most likely to come into contact with people, and lodestone marauders are quite aggressive creatures that take to carnivory just as readily as ferrovory, their primary diet.
Iron is the most readily consumed mineral and the most important one in a marauder's diet, as it is biomineralized into its armor. This creates a very difficult to penetrate shell and results in the rusty coloration that can be seen on them. Lodestone marauders can be distinguished in age based on the number of spikes covering their bodies, growing more as they mature. Another reason that city dwelling marauders are dangerous is because individuals who feed on worked metal seem to grow much faster than those that feed on natural deposits, likely due to the purity of the metal.
By far the most dangerous trait of a lodestone marauder however is not their metallic skin or powerful jaws or rending talons. No, it is their ability to create magnetic fields around themselves that can attract or repel metallic objects as they see fit. This is used to great effect against adventurers who may otherwise surround and slaughter the lumbering bulky beast. Marauders can attract metallic objects to their skin, brining their food to themselves without too much effort, or they can repel such objects away from themselves making for either a easy escape or leaving their opponent more or less defenseless for the kill. The attraction of metals to their skin also allows them to transport their food with them, until such a time as they feel they can eat uninterrupted.
Iron Bond. The most well accepted origin for lodestone marauders is that they were the result of magical experimentation, likely for the purposes of guardians of treasure which drow and duergar are known to use them for, but many of the original stock escaped their masters and turned feral. Luckily for the realms, lodestone marauders have simple, animalistic desires: food, space, and breeding.
A good deal of a lodestone marauder's time is spent searching for food, be it hunting whatever animals it can find or stocking up on caches of metal ore, and as such they are highly territorial of their range. Once a marauder finds a suitable territory with plentiful food, it establishes a den and gorges itself on the local bounty, fighting off any that would seek to take its territory from it especially other marauders. In general, the older individuals are larger and are therefore better able to defend a territory from younger ones.
The only exception to this rule is when it comes time to find a mate, for which they prepare by gathering up a large pile of metallic objects in their dens to present as a nuptial gift that shows of the quality of their territory. Both sexes are equally likely to do this, as dimorphism between them is so minimal that males and females are indistinguishable to observers. The actual courtship is extremely brief, but the bond is strong and lodestone marauders mate for life. After breeding, a female lays six eggs with an iron encrusted shell, which are tended to by both parents until they hatch two months later. The infants are cared for until they reach six months of age, at which point they leave their natal den to form a territory of their own and express no familial bonds to each other their after.