Clockwork Man

Immutable Form. The monster is immune to any spell or effect that would alter its form.

Inner Workings. The monster is vulnerable to oils and ice if they can pass its glass protection layer.

Bad Programming. Since the Clockwork Man detected living beings by their breathing, it could be tricked into thinking that a living being was a construct too, by holding their breath.

Magic Resistance. The monster has advantage on saving throws against spells and other magical effects.

Whirring & Spinning Gears. While moving, the monster has disadvantage on Dexterity (Stealth) checks.

Constructed Nature. A Clockwork Man doesn't require air, food, drink or sleep.

False Appearance. The monster followed camouflage procedures to blend into the surroundings. This could be as simple as dressing in appropriate costumes to match the time periods it found itself in — despite looking somewhat unrealistic. A more drastic procedure of camouflage was the harvesting of human skin and organs to outwardly appear human.

Invisible. If the Clockwork Man has not been seen for a minute it can choose to turn invisible. However, it produces the distinct "tick tock" sound of a clock but It breaks any clock in the room to avoid raising suspicion. In order for a creature to notice it, the creature needs to succeed on a Perception 19 DC ability check.

Repair. The monster is able to replace its tools with local spare parts, such as sword blades. It can also repair itself using parts it finds in its vicinity usually metalwork and organic parts like human organs. It could also heat itself if it is frozen and empty unwanted fluids from its system.

Telepathy. The monster is able to instantly communicate with any creature of its type (Clockwork Man, Clockwork Child etc.) basic ideas.

Multiattack. The monster makes two melee attacks.

Blades. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) slashing damage.

Rapier. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage.

Poison Dart. (2/dawn) Ranged Weapon Attack: +6 to hit, range 25/100, two targets within 10 feet of each other. Hit: 10 (2d6 +3) poison damage and the target needs to roll a DC 13 CON saving throw.  If the target fails, they are poisoned.

Tranquilizer Dart. (2/dawn) Ranged Weapon Attack: +6 to hit, range 25/100, two targets within 10 feet of each other. Hit: 3 (1d4 +1) piercing damage and the target needs to roll a DC 14 CON saving throw. If the target fails, they fall prone, drops anything it holds and is asleep until they take damage or regain hit points or a minute passes or at the start of their turn succeed on a DC 14 CON saving throw. 

Create Construct (1/Day). The monster magically summons one Clockwork Child, appearing within 5 feet of the monster. This creation lasts until destroyed and obeys every command given to it by the monster it was created from.

The Clockwork Child  differs from the Clockwork Man in the following aspects:

• has half the hit points
• its size is small
• its to hit modifiers are +3
• it does not have the tranquilizer and poison dart actions but has the Ticking Bomb. action

Ticking Bomb. When the clockwork child takes damage for the first time, it creates a synthetic, glowing bead of fire energy inside its glass container as a reaction. When the clockwork child drops to 0 hit points, the bead detonates in an explosion of flame that spreads around corners. Each creature in a 15-foot-radius sphere centered on that point must make a DC 12 Dexterity saving throw. A creature takes fire damage equal to the total accumulated damage on a failed save, or half as much damage on a successful one.
The explosion’s base damage is 2d6. If at the end of the clockwork child's turn the bead has not yet detonated, the damage increases by 1d6. The fire damages objects in the area and ignites flammable objects that aren’t being worn or carried.