Basically an Invisible Stalker with a little more Rust Monster.
The ruin tokens it leaves on items can be repaired with an appropriate kit during a short rest.
Hit Points 104 (16d8 + 32)
Speed 50 ft., fly 50 ft. (hover)
Skills Perception +8, Stealth +10
Damage Vulnerabilities force
Damage Resistances acid, cold, fire, lightning, thunder; nonmagical attacks
Damage Immunities necrotic, poison; nonmagical non-metallic ammunition
Condition Immunities exhaustion, paralyzed, petrified, poisoned, prone, unconscious
Senses darkvision 60 ft., passive Perception 18
Languages Terran, understands Common but doesn’t speak it
Challenge 6 (2300 XP)
Shadow Invisibility. The elemental is invisible in dim light or darkness.
Ammunition Ruin. Manufactured nonmagical ammunition which is not wholly made from stone or metal is disintegrated before dealing damage to the elemental. Thrown weapons and ammunition wholly of stone or metal (such as sling stones) are immune to this effect, but vulnerable to weapon ruin.
Weapon Ruin. Any manufactured weapon striking the elemental takes a cumulative -1 penalty to attack and damage after striking, nonmagical items being destroyed at -5. Attackers using natural weapons or unarmed strikes takes 5 (1d10) necrotic damage after striking instead.
Ruinous Body. Any creature continuously touching the elemental (such as with a grapple, regardless of initiator) takes 11 (2d10) necrotic damage at the end of their turn each round in which they remain in contact with the elemental. This also applies to nets, shackles, etc.
Multiattack. The elemental makes two ruinous slam attacks.
Ruinous Slam. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 8 (2d6 + 1) necrotic damage. This deals an additional 11 (2d10) necrotic damage to targets without armor or shield; otherwise, the target’s shield or armor takes a -1 penalty to AC, nonmagical items being destroyed at AC 10 (or 0 for shields).
Any living creature reduced to 0 hit points via a ruinous slam is destroyed and spawns sufficient swarms of ruin mites to fill their space (0 for a tiny or smaller creature, 1 for a small or medium creature, 4 for a large, 9 for a huge, etc).
Meanwhile, the ruin mites. They’re an insect swarm merged with a shadow but themed towards age and ruin. They probably grow into ruin elementals, eventually.
Medium swarm of tiny elementals, neutral
Hit Points 27 (5d8 + 5)
Speed 40 ft.
Skills Stealth +4 (+6 in dim light or darkness)
Damage Vulnerabilities force
Damage Resistances acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities necrotic, poison
Condition Immunities exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained
Senses darkvision 60 ft., passive Perception 10
Challenge 1 (200 XP)
Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny insect. The swarm can’t regain hit points or gain temporary hit points. Any creature ending its turn in the swarm’s space takes 5 (1d10) necrotic damage and loses 5 pounds of soft goods to ruin and spoilage: leather, spellbooks, scrolls, cloth, rope, rations, potions, alchemical gear.
Shadow Invisibility. The swarm is invisible in dim light or in darkness.
Ruin. Melee Spell Attack: reach 5 ft., one creature. DC 10 Wisdom save. Success: The creature is immune to all effects except for damage from this effect for the next 24 hours. Failure while the swarm is above half hit points: 11 (2d10) necrotic damage and the target ages half that amount in years. Failure while the swarm is below half hit points: 5 (1d10) necrotic damage, and the target ages 1 year. The aging effect may be reversed with a greater restoration effect cast within 24 hours.